Summary of Significant Accounting Policies - Effects of Reinsurance (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
Premiums Written
Direct									$ 3,654,200	$ 3,774,600	$ 3,550,900
Net									3,610,000	3,730,000	3,500,000
Premiums Earned
Direct									3,603,780	3,655,233	3,416,687
Ceded									(56,212)	(56,725)	(48,941)
Assumed									8,067	910	665
Net	$ 921,860	$ 899,304	$ 811,897	$ 922,574	$ 925,384	$ 915,012	$ 888,776	$ 870,245	3,555,635	3,599,418	3,368,411
Segment Reporting Information [Line Items]
Direct premiums written									3,654,200	3,774,600	3,550,900
Net									3,610,000	3,730,000	3,500,000
Direct amounts									3,603,780	3,655,233	3,416,687
Ceded premiums									56,212	56,725	48,941
Assumed									8,067	910	665
Net premiums earned	$ 921,860	$ 899,304	$ 811,897	$ 922,574	$ 925,384	$ 915,012	$ 888,776	$ 870,245	3,555,635	3,599,418	3,368,411
Property and Casualty Lines [Member]
Premiums Written
Direct									3,624,965	3,742,627	3,524,186
Ceded									(50,630)	(43,785)	(55,973)
Assumed									8,012	919	689
Net									3,582,347	3,699,761	3,468,902
Premiums Earned
Direct									3,574,898	3,626,857	3,386,192
Ceded									(56,212)	(56,725)	(48,941)
Assumed									8,067	910	665
Net									3,526,753	3,571,042	3,337,916
Segment Reporting Information [Line Items]
Direct premiums written									3,624,965	3,742,627	3,524,186
Ceded									50,630	43,785	55,973
Assumed									8,012	919	689
Net									3,582,347	3,699,761	3,468,902
Direct amounts									3,574,898	3,626,857	3,386,192
Ceded premiums									56,212	56,725	48,941
Assumed									8,067	910	665
Net premiums earned									$ 3,526,753	$ 3,571,042	$ 3,337,916