Summary of Investments - Fair Value of Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	$ 1,662,680	$ 1,656,797
Gross Unrealized Appreciation	2,784	71,642
Gross Unrealized Depreciation	114,125	8,649
Fair Value	1,551,339	1,719,790
Allowance for Credit Losses	3	0
Carrying Value	1,551,336	1,719,790
U.S. Treasury
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	15,684	42,425
Gross Unrealized Appreciation	0	216
Gross Unrealized Depreciation	1,009	718
Fair Value	14,675	41,923
Allowance for Credit Losses	0	0
Carrying Value	14,675	41,923
U.S. government agency
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	94,092	60,074
Gross Unrealized Appreciation	35	2,155
Gross Unrealized Depreciation	9,721	562
Fair Value	84,406	61,667
Allowance for Credit Losses	0	0
Carrying Value	84,406	61,667
States, municipalities and political subdivisions | General obligations: | Midwest
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	61,191	71,863
Gross Unrealized Appreciation	185	2,483
Gross Unrealized Depreciation	263	0
Fair Value	61,113	74,346
Allowance for Credit Losses	0	0
Carrying Value	61,113	74,346
States, municipalities and political subdivisions | General obligations: | Northeast
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	15,518	22,061
Gross Unrealized Appreciation	18	701
Gross Unrealized Depreciation	73	0
Fair Value	15,463	22,762
Allowance for Credit Losses	0	0
Carrying Value	15,463	22,762
States, municipalities and political subdivisions | General obligations: | South
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	64,851	90,171
Gross Unrealized Appreciation	57	3,873
Gross Unrealized Depreciation	927	0
Fair Value	63,981	94,044
Allowance for Credit Losses	0	0
Carrying Value	63,981	94,044
States, municipalities and political subdivisions | General obligations: | West
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	87,094	93,968
Gross Unrealized Appreciation	163	5,110
Gross Unrealized Depreciation	712	0
Fair Value	86,545	99,078
Allowance for Credit Losses	0	0
Carrying Value	86,545	99,078
States, municipalities and political subdivisions | Special revenue: | Midwest
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	103,107	114,997
Gross Unrealized Appreciation	224	7,292
Gross Unrealized Depreciation	1,065	0
Fair Value	102,266	122,289
Allowance for Credit Losses	0	0
Carrying Value	102,266	122,289
States, municipalities and political subdivisions | Special revenue: | Northeast
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	55,292	55,811
Gross Unrealized Appreciation	76	3,921
Gross Unrealized Depreciation	1,148	0
Fair Value	54,220	59,732
Allowance for Credit Losses	0	0
Carrying Value	54,220	59,732
States, municipalities and political subdivisions | Special revenue: | South
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	184,108	201,383
Gross Unrealized Appreciation	278	14,365
Gross Unrealized Depreciation	3,529	78
Fair Value	180,857	215,670
Allowance for Credit Losses	0	0
Carrying Value	180,857	215,670
States, municipalities and political subdivisions | Special revenue: | West
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	113,594	126,521
Gross Unrealized Appreciation	275	8,128
Gross Unrealized Depreciation	1,657	0
Fair Value	112,212	134,649
Allowance for Credit Losses	0	0
Carrying Value	112,212	134,649
Foreign bonds
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	36,129	30,314
Gross Unrealized Appreciation	0	789
Gross Unrealized Depreciation	4,480	197
Fair Value	31,649	30,906
Allowance for Credit Losses	0	0
Carrying Value	31,649	30,906
Public utilities
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	138,752	104,008
Gross Unrealized Appreciation	65	3,966
Gross Unrealized Depreciation	13,406	481
Fair Value	125,411	107,493
Allowance for Credit Losses	0	0
Carrying Value	125,411	107,493
Corporate bonds | Energy
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	36,507	31,011
Gross Unrealized Appreciation	0	1,751
Gross Unrealized Depreciation	3,298	81
Fair Value	33,209	32,681
Allowance for Credit Losses	0	0
Carrying Value	33,209	32,681
Corporate bonds | Industrials
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	58,334	55,014
Gross Unrealized Appreciation	62	2,319
Gross Unrealized Depreciation	5,554	162
Fair Value	52,842	57,171
Allowance for Credit Losses	0	0
Carrying Value	52,842	57,171
Corporate bonds | Consumer goods and services
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	100,539	71,543
Gross Unrealized Appreciation	0	1,912
Gross Unrealized Depreciation	10,598	611
Fair Value	89,941	72,844
Allowance for Credit Losses	0	0
Carrying Value	89,941	72,844
Corporate bonds | Health care
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	32,987	27,351
Gross Unrealized Appreciation	24	539
Gross Unrealized Depreciation	5,419	461
Fair Value	27,592	27,429
Allowance for Credit Losses	0	0
Carrying Value	27,592	27,429
Corporate bonds | Technology, media and telecommunications
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	67,193	55,405
Gross Unrealized Appreciation	0	2,958
Gross Unrealized Depreciation	7,253	866
Fair Value	59,940	57,497
Allowance for Credit Losses	0	0
Carrying Value	59,940	57,497
Corporate bonds | Financial services
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	132,849	98,352
Gross Unrealized Appreciation	851	4,394
Gross Unrealized Depreciation	9,408	131
Fair Value	124,292	102,615
Allowance for Credit Losses	3	0
Carrying Value	124,289	102,615
Mortgage-backed securities
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	20,450	25,075
Gross Unrealized Appreciation	0	167
Gross Unrealized Depreciation	2,750	229
Fair Value	17,700	25,013
Allowance for Credit Losses	0	0
Carrying Value	17,700	25,013
Collateralized mortgage obligations | Government National Mortgage Association
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	97,839	109,968
Gross Unrealized Appreciation	0	2,322
Gross Unrealized Depreciation	13,291	1,772
Fair Value	84,548	110,518
Allowance for Credit Losses	0	0
Carrying Value	84,548	110,518
Collateralized mortgage obligations | Federal Home Loan Mortgage Corporation
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	92,366	120,911
Gross Unrealized Appreciation	0	736
Gross Unrealized Depreciation	13,528	1,658
Fair Value	78,838	119,989
Allowance for Credit Losses	0	0
Carrying Value	78,838	119,989
Collateralized mortgage obligations | Federal National Mortgage Association
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	50,272	48,246
Gross Unrealized Appreciation	5	945
Gross Unrealized Depreciation	4,891	642
Fair Value	45,386	48,549
Allowance for Credit Losses	0	0
Carrying Value	45,386	48,549
Asset-backed securities
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	3,932	325
Gross Unrealized Appreciation	466	600
Gross Unrealized Depreciation	145	0
Fair Value	4,253	925
Allowance for Credit Losses	0	0
Carrying Value	4,253	925
Redeemable preferred stocks
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	0	0
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	0	0
Fair Value	0	0
Allowance for Credit Losses	0	0
Carrying Value		$ 0