Investments - Gross Unrealized Losses and Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Marketable Securities
Estimated Fair Value, Less than 12 Months	$ 1,006,337	$ 369,954
Estimated Fair Value, 12 Months or More	131,303	13,488
Estimated Fair Value, Total	1,137,640	383,442
Unrealized Losses, Less than 12 Months	116,972	6,983
Unrealized Losses, 12 Months or More	28,367	591
Unrealized Losses, Total	145,339	7,574
Subtotal, fixed maturity securities
Fixed Maturity Securities
Estimated Fair Value, Less than 12 Months	877,343	350,497
Estimated Fair Value, 12 Months or More	127,894	12,459
Estimated Fair Value, Total	1,005,237	362,956
Unrealized Losses, Less than 12 Months	88,769	5,424
Unrealized Losses, 12 Months or More	26,320	386
Unrealized Losses, Total	115,089	5,810
U.S. Treasury securities
Fixed Maturity Securities
Estimated Fair Value, Less than 12 Months	1,657
Estimated Fair Value, Total	1,657
Unrealized Losses, Less than 12 Months	169
Unrealized Losses, Total	169
Obligations of states and political subdivisions
Fixed Maturity Securities
Estimated Fair Value, Less than 12 Months	44,270	2,985
Estimated Fair Value, 12 Months or More	2,154	1,012
Estimated Fair Value, Total	46,424	3,997
Unrealized Losses, Less than 12 Months	3,425	85
Unrealized Losses, 12 Months or More	817	38
Unrealized Losses, Total	4,242	123
Residential mortgage-backed securities
Fixed Maturity Securities
Estimated Fair Value, Less than 12 Months	193,570	97,116
Estimated Fair Value, 12 Months or More	22,269	11
Estimated Fair Value, Total	215,839	97,127
Unrealized Losses, Less than 12 Months	24,470	1,502
Unrealized Losses, 12 Months or More	4,820	1
Unrealized Losses, Total	29,290	1,503
Commercial mortgage-backed securities
Fixed Maturity Securities
Estimated Fair Value, Less than 12 Months	128,526	29,660
Estimated Fair Value, 12 Months or More	8,212
Estimated Fair Value, Total	136,738	29,660
Unrealized Losses, Less than 12 Months	12,991	442
Unrealized Losses, 12 Months or More	2,123
Unrealized Losses, Total	15,114	442
Other asset-backed securities
Fixed Maturity Securities
Estimated Fair Value, Less than 12 Months	65,183	39,266
Estimated Fair Value, 12 Months or More	4,292
Estimated Fair Value, Total	69,475	39,266
Unrealized Losses, Less than 12 Months	4,955	255
Unrealized Losses, 12 Months or More	193
Unrealized Losses, Total	5,148	255
Corporate and other securities
Fixed Maturity Securities
Estimated Fair Value, Less than 12 Months	444,137	181,470
Estimated Fair Value, 12 Months or More	90,967	11,436
Estimated Fair Value, Total	535,104	192,906
Unrealized Losses, Less than 12 Months	42,759	3,140
Unrealized Losses, 12 Months or More	18,367	347
Unrealized Losses, Total	61,126	3,487
Equity securities
Equity Securities
Estimated Fair Value, Less than 12 Months	128,994	19,457
Estimated Fair Value, 12 Months or More	3,409	1,029
Estimated Fair Value, Total	132,403	20,486
Unrealized Losses, Less than 12 Months	28,203	1,559
Unrealized Losses, 12 Months or More	2,047	205
Unrealized Losses, Total	$ 30,250	$ 1,764