Segment Information (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Lease revenues									$ 198,121	$ 217,230	$ 201,112
Operating revenues	$ 213,531	$ 175,414	$ 172,585	$ 192,296	$ 192,761	$ 200,658	$ 197,023	$ 209,524	753,826	799,966	835,750
Voyage charters
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									60,863	46,381	73,979
Contracts of affreightment
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									211,887	226,857	231,247
Tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									68,758	79,757	74,157
Unit freight
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									48,934	63,420	58,326
Terminal operations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									20,794	16,562	21,501
Fleeting operations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									17,993	17,264	17,888
Logistics services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									15,892	15,155	14,309
Time and material contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									78,184	87,167	119,196
Retainer contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									11,823	9,874	10,124
Product sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									6,402	5,555	2,686
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									14,175	14,744	11,225
Ocean Services
Disaggregation of Revenue [Line Items]
Operating revenues									381,255	423,288	414,844
Inland Services
Disaggregation of Revenue [Line Items]
Operating revenues									271,889	267,334	285,688
Witt O’Brien’s
Disaggregation of Revenue [Line Items]
Operating revenues									92,416	101,663	131,629
Other
Disaggregation of Revenue [Line Items]
Operating revenues									8,266	7,681	3,589
Operating Segments \| Ocean Services
Disaggregation of Revenue [Line Items]
Lease revenues									185,872	205,346	193,242
Operating revenues									381,255	423,288	414,844
Operating Segments \| Ocean Services \| Voyage charters
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									60,863	46,381	73,979
Operating Segments \| Ocean Services \| Contracts of affreightment
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									12,876	24,540	11,872
Operating Segments \| Ocean Services \| Tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									68,758	79,757	74,157
Operating Segments \| Ocean Services \| Unit freight
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									48,934	63,420	58,326
Operating Segments \| Ocean Services \| Terminal operations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Ocean Services \| Fleeting operations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Ocean Services \| Logistics services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Ocean Services \| Time and material contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Ocean Services \| Retainer contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Ocean Services \| Product sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Ocean Services \| Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									3,952	3,844	3,268
Operating Segments \| Inland Services
Disaggregation of Revenue [Line Items]
Lease revenues									11,850	10,902	7,392
Operating revenues									271,889	267,334	285,688
Operating Segments \| Inland Services \| Voyage charters
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Inland Services \| Contracts of affreightment
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									199,011	202,317	219,375
Operating Segments \| Inland Services \| Tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Inland Services \| Unit freight
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Inland Services \| Terminal operations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									20,794	16,562	21,501
Operating Segments \| Inland Services \| Fleeting operations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									17,993	17,264	17,888
Operating Segments \| Inland Services \| Logistics services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									15,892	15,155	14,309
Operating Segments \| Inland Services \| Time and material contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Inland Services \| Retainer contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Inland Services \| Product sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Inland Services \| Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									6,349	5,134	5,223
Operating Segments \| Witt O’Brien’s
Disaggregation of Revenue [Line Items]
Lease revenues									0	0	0
Operating revenues									92,480	101,783	131,709
Operating Segments \| Witt O’Brien’s \| Voyage charters
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Witt O’Brien’s \| Contracts of affreightment
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Witt O’Brien’s \| Tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Witt O’Brien’s \| Unit freight
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Witt O’Brien’s \| Terminal operations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Witt O’Brien’s \| Fleeting operations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Witt O’Brien’s \| Logistics services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Witt O’Brien’s \| Time and material contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									78,184	87,167	119,196
Operating Segments \| Witt O’Brien’s \| Retainer contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									11,823	9,874	10,124
Operating Segments \| Witt O’Brien’s \| Product sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Witt O’Brien’s \| Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									2,473	4,742	2,389
Operating Segments \| Other
Disaggregation of Revenue [Line Items]
Lease revenues									399	982	478
Operating revenues									8,266	7,681	3,589
Operating Segments \| Other \| Voyage charters
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Other \| Contracts of affreightment
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Other \| Tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Other \| Unit freight
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Other \| Terminal operations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Other \| Fleeting operations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Other \| Logistics services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Other \| Time and material contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Other \| Retainer contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Operating Segments \| Other \| Product sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									6,402	5,555	2,686
Cost of goods and services sold									4,500	4,500	2,100
Operating Segments \| Other \| Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									1,465	1,144	425
Corporate And Eliminations
Disaggregation of Revenue [Line Items]
Lease revenues									0	0	0
Operating revenues									(64)	(120)	(80)
Corporate And Eliminations \| Voyage charters
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Corporate And Eliminations \| Contracts of affreightment
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Corporate And Eliminations \| Tariff
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Corporate And Eliminations \| Unit freight
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Corporate And Eliminations \| Terminal operations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Corporate And Eliminations \| Fleeting operations
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Corporate And Eliminations \| Logistics services
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Corporate And Eliminations \| Time and material contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Corporate And Eliminations \| Retainer contracts
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Corporate And Eliminations \| Product sales
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									0	0	0
Corporate And Eliminations \| Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers									$ (64)	$ (120)	$ (80)

SEACOR HOLDINGS INC /NEW/ 10-K