Summary of Investments - Fair Value of Investments (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	$ 1,771,041	$ 1,662,680
Gross Unrealized Appreciation	6,810	2,784
Gross Unrealized Depreciation	91,348	114,125
Fair Value	1,686,503	1,551,339
Allowance for Credit Losses	1	3
Carrying Value	1,686,502	1,551,336
U.S. Treasury
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	51,211	15,684
Gross Unrealized Appreciation	325	0
Gross Unrealized Depreciation	675	1,009
Fair Value	50,861	14,675
Allowance for Credit Losses	0	0
Carrying Value	50,861	14,675
U.S. government agency
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	102,540	94,092
Gross Unrealized Appreciation	255	35
Gross Unrealized Depreciation	8,302	9,721
Fair Value	94,493	84,406
Allowance for Credit Losses	0	0
Carrying Value	94,493	84,406
States, municipalities and political subdivisions | General obligations: | Midwest
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	52,712	61,191
Gross Unrealized Appreciation	132	185
Gross Unrealized Depreciation	137	263
Fair Value	52,707	61,113
Allowance for Credit Losses	0	0
Carrying Value	52,707	61,113
States, municipalities and political subdivisions | General obligations: | Northeast
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	11,422	15,518
Gross Unrealized Appreciation	1	18
Gross Unrealized Depreciation	43	73
Fair Value	11,380	15,463
Allowance for Credit Losses	0	0
Carrying Value	11,380	15,463
States, municipalities and political subdivisions | General obligations: | South
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	54,560	64,851
Gross Unrealized Appreciation	47	57
Gross Unrealized Depreciation	400	927
Fair Value	54,207	63,981
Allowance for Credit Losses	0	0
Carrying Value	54,207	63,981
States, municipalities and political subdivisions | General obligations: | West
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	77,874	87,094
Gross Unrealized Appreciation	23	163
Gross Unrealized Depreciation	471	712
Fair Value	77,426	86,545
Allowance for Credit Losses	0	0
Carrying Value	77,426	86,545
States, municipalities and political subdivisions | Special revenue: | Midwest
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	101,037	103,107
Gross Unrealized Appreciation	302	224
Gross Unrealized Depreciation	358	1,065
Fair Value	100,981	102,266
Allowance for Credit Losses	0	0
Carrying Value	100,981	102,266
States, municipalities and political subdivisions | Special revenue: | Northeast
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	52,708	55,292
Gross Unrealized Appreciation	79	76
Gross Unrealized Depreciation	560	1,148
Fair Value	52,227	54,220
Allowance for Credit Losses	0	0
Carrying Value	52,227	54,220
States, municipalities and political subdivisions | Special revenue: | South
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	166,119	184,108
Gross Unrealized Appreciation	302	278
Gross Unrealized Depreciation	2,155	3,529
Fair Value	164,266	180,857
Allowance for Credit Losses	0	0
Carrying Value	164,266	180,857
States, municipalities and political subdivisions | Special revenue: | West
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	102,254	113,594
Gross Unrealized Appreciation	147	275
Gross Unrealized Depreciation	836	1,657
Fair Value	101,565	112,212
Allowance for Credit Losses	0	0
Carrying Value	101,565	112,212
Foreign bonds
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	21,255	36,129
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	2,083	4,480
Fair Value	19,172	31,649
Allowance for Credit Losses	0	0
Carrying Value	19,172	31,649
Public utilities
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	149,734	138,752
Gross Unrealized Appreciation	787	65
Gross Unrealized Depreciation	10,054	13,406
Fair Value	140,467	125,411
Allowance for Credit Losses	0	0
Carrying Value	140,467	125,411
Corporate bonds | Energy
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	45,351	36,507
Gross Unrealized Appreciation	249	0
Gross Unrealized Depreciation	2,127	3,298
Fair Value	43,473	33,209
Allowance for Credit Losses	0	0
Carrying Value	43,473	33,209
Corporate bonds | Industrials
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	74,760	58,334
Gross Unrealized Appreciation	727	62
Gross Unrealized Depreciation	4,939	5,554
Fair Value	70,548	52,842
Allowance for Credit Losses	0	0
Carrying Value	70,548	52,842
Corporate bonds | Consumer goods and services
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	103,315	100,539
Gross Unrealized Appreciation	271	0
Gross Unrealized Depreciation	7,665	10,598
Fair Value	95,921	89,941
Allowance for Credit Losses	0	0
Carrying Value	95,921	89,941
Corporate bonds | Health care
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	37,872	32,987
Gross Unrealized Appreciation	99	24
Gross Unrealized Depreciation	4,499	5,419
Fair Value	33,472	27,592
Allowance for Credit Losses	0	0
Carrying Value	33,472	27,592
Corporate bonds | Technology, media and telecommunications
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	87,002	67,193
Gross Unrealized Appreciation	451	0
Gross Unrealized Depreciation	5,665	7,253
Fair Value	81,788	59,940
Allowance for Credit Losses	0	0
Carrying Value	81,788	59,940
Corporate bonds | Financial services
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	152,329	132,849
Gross Unrealized Appreciation	743	851
Gross Unrealized Depreciation	7,381	9,408
Fair Value	145,691	124,292
Allowance for Credit Losses	1	3
Carrying Value	145,690	124,289
Mortgage-backed securities
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	23,800	20,450
Gross Unrealized Appreciation	11	0
Gross Unrealized Depreciation	2,328	2,750
Fair Value	21,483	17,700
Allowance for Credit Losses	0	0
Carrying Value	21,483	17,700
Collateralized mortgage obligations | Government National Mortgage Association
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	164,666	97,839
Gross Unrealized Appreciation	1,282	0
Gross Unrealized Depreciation	12,742	13,291
Fair Value	153,206	84,548
Allowance for Credit Losses	0	0
Carrying Value	153,206	84,548
Collateralized mortgage obligations | Federal Home Loan Mortgage Corporation
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	84,842	92,366
Gross Unrealized Appreciation	20	0
Gross Unrealized Depreciation	13,177	13,528
Fair Value	71,685	78,838
Allowance for Credit Losses	0	0
Carrying Value	71,685	78,838
Collateralized mortgage obligations | Federal National Mortgage Association
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	50,284	50,272
Gross Unrealized Appreciation	33	5
Gross Unrealized Depreciation	4,664	4,891
Fair Value	45,653	45,386
Allowance for Credit Losses	0	0
Carrying Value	45,653	45,386
Asset-backed securities
Debt Securities, Available-for-Sale [Abstract]
Cost or Amortized Cost	3,394	3,932
Gross Unrealized Appreciation	524	466
Gross Unrealized Depreciation	87	145
Fair Value	3,831	4,253
Allowance for Credit Losses	0	0
Carrying Value	$ 3,831	$ 4,253