Investments - Gross Unrealized Losses and Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Marketable Securities
Estimated Fair Value, Less than 12 Months	$ 781,201	$ 369,954
Estimated Fair Value, 12 Months or More	46,030	13,488
Estimated Fair Value, Total	827,231	383,442
Unrealized Losses, Less than 12 Months	37,463	6,983
Unrealized Losses, 12 Months or More	4,448	591
Unrealized Losses, Total	41,911	7,574
Subtotal, fixed maturity securities
Marketable Securities
Estimated Fair Value, Less than 12 Months	718,280	350,497
Estimated Fair Value, 12 Months or More	45,677	12,459
Estimated Fair Value, Total	763,957	362,956
Unrealized Losses, Less than 12 Months	33,007	5,424
Unrealized Losses, 12 Months or More	4,100	386
Unrealized Losses, Total	37,107	5,810
U.S. Treasury securities
Marketable Securities
Estimated Fair Value, Less than 12 Months	1,756
Estimated Fair Value, Total	1,756
Unrealized Losses, Less than 12 Months	71
Unrealized Losses, Total	71
Obligations of states and political subdivisions
Marketable Securities
Estimated Fair Value, Less than 12 Months	19,793	2,985
Estimated Fair Value, 12 Months or More	904	1,012
Estimated Fair Value, Total	20,697	3,997
Unrealized Losses, Less than 12 Months	1,214	85
Unrealized Losses, 12 Months or More	146	38
Unrealized Losses, Total	1,360	123
Residential mortgage-backed securities
Marketable Securities
Estimated Fair Value, Less than 12 Months	164,056	97,116
Estimated Fair Value, 12 Months or More	6,657	11
Estimated Fair Value, Total	170,713	97,127
Unrealized Losses, Less than 12 Months	8,366	1,502
Unrealized Losses, 12 Months or More	653	1
Unrealized Losses, Total	9,019	1,503
Commercial mortgage-backed securities
Marketable Securities
Estimated Fair Value, Less than 12 Months	118,414	29,660
Estimated Fair Value, 12 Months or More	5,339
Estimated Fair Value, Total	123,753	29,660
Unrealized Losses, Less than 12 Months	4,304	442
Unrealized Losses, 12 Months or More	710
Unrealized Losses, Total	5,014	442
Other asset-backed securities
Marketable Securities
Estimated Fair Value, Less than 12 Months	60,662	39,266
Estimated Fair Value, Total	60,662	39,266
Unrealized Losses, Less than 12 Months	2,091	255
Unrealized Losses, Total	2,091	255
Corporate and other securities
Marketable Securities
Estimated Fair Value, Less than 12 Months	353,599	181,470
Estimated Fair Value, 12 Months or More	32,777	11,436
Estimated Fair Value, Total	386,376	192,906
Unrealized Losses, Less than 12 Months	16,961	3,140
Unrealized Losses, 12 Months or More	2,591	347
Unrealized Losses, Total	19,552	3,487
Equity securities
Equity Securities
Estimated Fair Value, Less than 12 Months	62,921	19,457
Estimated Fair Value, 12 Months or More	353	1,029
Estimated Fair Value, Total	63,274	20,486
Unrealized Losses, Less than 12 Months	4,456	1,559
Unrealized Losses, 12 Months or More	348	205
Unrealized Losses, Total	$ 4,804	$ 1,764