Investments - Continuous loss position investments (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	996	722
Maximum
Schedule of Available-for-sale Securities [Line Items]
Percentage of book value (percent)	95.00%
Minimum
Schedule of Available-for-sale Securities [Line Items]
Unrealized loss amount	$ 100,000
Less than 12 Months
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	680	398
Less than 12 Months, Fair Value	$ 849,333,000	$ 489,771,000
Less than 12 Months, Gross Unrealized Losses	$ (11,252,000)	$ (3,233,000)
Unrealized Losses as Percentage of Cost	1.30%	0.70%
Less than 12 Months \| Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	680	398
Less than 12 Months, Fair Value	$ 849,333,000	$ 489,771,000
Less than 12 Months, Gross Unrealized Losses	$ (11,252,000)	$ (3,233,000)
Unrealized Losses as Percentage of Cost	1.30%	0.70%
Less than 12 Months \| U.S. government
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	28	25
Less than 12 Months, Fair Value	$ 48,202,000	$ 46,160,000
Less than 12 Months, Gross Unrealized Losses	$ (881,000)	$ (422,000)
Unrealized Losses as Percentage of Cost	1.80%	0.90%
Less than 12 Months \| State and Municipal
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	99	82
Less than 12 Months, Fair Value	$ 184,046,000	$ 163,997,000
Less than 12 Months, Gross Unrealized Losses	$ (2,147,000)	$ (939,000)
Unrealized Losses as Percentage of Cost	1.20%	0.60%
Less than 12 Months \| Mortgage- Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	278	156
Less than 12 Months, Fair Value	$ 174,756,000	$ 85,419,000
Less than 12 Months, Gross Unrealized Losses	$ (2,427,000)	$ (483,000)
Unrealized Losses as Percentage of Cost	1.40%	0.60%
Less than 12 Months \| Residential
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	263	154
Less than 12 Months, Fair Value	$ 144,098,000	$ 81,841,000
Less than 12 Months, Gross Unrealized Losses	$ (2,250,000)	$ (453,000)
Unrealized Losses as Percentage of Cost	1.50%	0.60%
Less than 12 Months \| Commercial
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	15	2
Less than 12 Months, Fair Value	$ 30,658,000	$ 3,578,000
Less than 12 Months, Gross Unrealized Losses	$ (177,000)	$ (30,000)
Unrealized Losses as Percentage of Cost	0.60%	0.80%
Less than 12 Months \| Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	43	31
Less than 12 Months, Fair Value	$ 74,670,000	$ 35,407,000
Less than 12 Months, Gross Unrealized Losses	$ (628,000)	$ (193,000)
Unrealized Losses as Percentage of Cost	0.80%	0.50%
Less than 12 Months \| Corporates
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	232	104
Less than 12 Months, Fair Value	$ 367,660,000	$ 158,788,000
Less than 12 Months, Gross Unrealized Losses	$ (5,168,000)	$ (1,197,000)
Unrealized Losses as Percentage of Cost	1.40%	0.70%
Less than 12 Months \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security		0
Less than 12 Months, Fair Value		$ 0
Less than 12 Months, Gross Unrealized Losses		$ 0
Unrealized Losses as Percentage of Cost		0.00%
Less than 12 Months \| Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	0	0
Less than 12 Months, Fair Value	$ 0	$ 0
Less than 12 Months, Gross Unrealized Losses	$ 0	$ 0
Unrealized Losses as Percentage of Cost	0.00%	0.00%
12 Months or More
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	316	324
Unrealized Losses as Percentage of Cost	3.90%	2.40%
12 Months or More, Fair Value	$ 179,380,000	$ 193,309,000
12 Months of More, Gross Unrealized Losses	$ (7,369,000)	$ (4,809,000)
12 Months or More \| Fixed Maturities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	316	324
Unrealized Losses as Percentage of Cost	3.90%	2.40%
12 Months or More, Fair Value	$ 179,380,000	$ 193,309,000
12 Months of More, Gross Unrealized Losses	$ (7,369,000)	$ (4,809,000)
12 Months or More \| U.S. government
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	16	16
Unrealized Losses as Percentage of Cost	2.40%	1.70%
12 Months or More, Fair Value	$ 12,743,000	$ 14,368,000
12 Months of More, Gross Unrealized Losses	$ (307,000)	$ (246,000)
12 Months or More \| State and Municipal
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	4	5
Unrealized Losses as Percentage of Cost	1.40%	0.60%
12 Months or More, Fair Value	$ 4,573,000	$ 10,529,000
12 Months of More, Gross Unrealized Losses	$ (64,000)	$ (60,000)
12 Months or More \| Mortgage- Backed Securities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	280	288
Unrealized Losses as Percentage of Cost	4.40%	2.80%
12 Months or More, Fair Value	$ 142,480,000	$ 150,383,000
12 Months of More, Gross Unrealized Losses	$ (6,569,000)	$ (4,267,000)
12 Months or More \| Residential
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	272	279
Unrealized Losses as Percentage of Cost	4.60%	2.80%
12 Months or More, Fair Value	$ 119,273,000	$ 127,317,000
12 Months of More, Gross Unrealized Losses	$ (5,730,000)	$ (3,644,000)
12 Months or More \| Commercial
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	8	9
Unrealized Losses as Percentage of Cost	3.50%	2.60%
12 Months or More, Fair Value	$ 23,206,000	$ 23,066,000
12 Months of More, Gross Unrealized Losses	$ (839,000)	$ (623,000)
12 Months or More \| Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	1	2
Unrealized Losses as Percentage of Cost	0.10%	0.20%
12 Months or More, Fair Value	$ 1,458,000	$ 1,561,000
12 Months of More, Gross Unrealized Losses	$ (2,000)	$ (3,000)
12 Months or More \| Corporates
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	15	13
Unrealized Losses as Percentage of Cost	2.30%	1.40%
12 Months or More, Fair Value	$ 18,126,000	$ 16,468,000
12 Months of More, Gross Unrealized Losses	$ (427,000)	$ (232,000)
12 Months or More \| Equity securities
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security		0
Unrealized Losses as Percentage of Cost		0.00%
12 Months or More, Fair Value		$ 0
12 Months of More, Gross Unrealized Losses		$ 0
12 Months or More \| Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Number of Securities with Unrealized Losses (positions) \| security	0	0
Unrealized Losses as Percentage of Cost	0.00%	0.00%
12 Months or More, Fair Value	$ 0	$ 0
12 Months of More, Gross Unrealized Losses	$ 0	$ 0

INFINITY PROPERTY & CASUALTY CORP 10-Q