Segment Reporting - Geographic Schedule of Revenue and Long-lived Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	$ 368,253	$ 559,591
Long-lived assets	42,668	47,439
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	250,351	360,748
Long-lived assets	34,208	38,482
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	25,962	42,468
Long-lived assets	2,228	2,508
Mexico
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	34,466	49,915
Long-lived assets	3,202	3,233
France
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	28,026	53,644
Long-lived assets	1,781	1,961
China
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	21,186	34,897
Long-lived assets	472	374
All other countries
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	8,262	17,919
Long-lived assets	$ 777	$ 881

A. M. Castle & Co. 10-K