Summary of Investments (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fixed maturities:
Amortized Cost	$ 1,742,282	$ 1,720,291
Gross Unrealized Appreciation	86,588
Gross Unrealized Depreciation	11,127
Fair Value	1,816,765	1,825,438
Allowance for Credit Losses	174	5
Short-Term Investments
Fixed maturities:
Amortized Cost	801	175
Gross Unrealized Appreciation	3
Gross Unrealized Depreciation	0
Allowance for Credit Losses	0
Fixed maturities
Fixed maturities:
Amortized Cost	1,741,481	1,720,291
Gross Unrealized Appreciation	86,585	107,003
Gross Unrealized Depreciation	11,127	1,851
Allowance for Credit Losses	174	5
U.S. Treasury
Fixed maturities:
Amortized Cost	70,254	149,481
Gross Unrealized Appreciation	343	482
Gross Unrealized Depreciation	505	25
Allowance for Credit Losses	0	0
U.S. government agency
Fixed maturities:
Amortized Cost	87,150	60,502
Gross Unrealized Appreciation	3,265	4,016
Gross Unrealized Depreciation	2,258	0
Allowance for Credit Losses	0	0
States, municipalities and political subdivisions | General obligations | Midwest
Fixed maturities:
Amortized Cost	68,578	77,933
Gross Unrealized Appreciation	2,951	4,047
Gross Unrealized Depreciation	0	0
Allowance for Credit Losses	0	0
States, municipalities and political subdivisions | General obligations | Northeast
Fixed maturities:
Amortized Cost	29,018	29,071
Gross Unrealized Appreciation	1,081	1,379
Gross Unrealized Depreciation	0	0
Allowance for Credit Losses	0	0
States, municipalities and political subdivisions | General obligations | South
Fixed maturities:
Amortized Cost	94,493	104,522
Gross Unrealized Appreciation	4,695	5,448
Gross Unrealized Depreciation	0	0
Allowance for Credit Losses	0	0
States, municipalities and political subdivisions | General obligations | West
Fixed maturities:
Amortized Cost	97,459	102,590
Gross Unrealized Appreciation	6,042	7,431
Gross Unrealized Depreciation	0	0
Allowance for Credit Losses	0	0
States, municipalities and political subdivisions | Special revenue | Midwest
Fixed maturities:
Amortized Cost	123,401	115,956
Gross Unrealized Appreciation	8,257	9,142
Gross Unrealized Depreciation	0	0
Allowance for Credit Losses	0	0
States, municipalities and political subdivisions | Special revenue | Northeast
Fixed maturities:
Amortized Cost	56,193	56,317
Gross Unrealized Appreciation	3,972	4,759
Gross Unrealized Depreciation	0	0
Allowance for Credit Losses	0	0
States, municipalities and political subdivisions | Special revenue | South
Fixed maturities:
Amortized Cost	211,100	208,739
Gross Unrealized Appreciation	15,728	17,967
Gross Unrealized Depreciation	98	0
Allowance for Credit Losses	0	0
States, municipalities and political subdivisions | Special revenue | West
Fixed maturities:
Amortized Cost	131,711	129,417
Gross Unrealized Appreciation	8,375	9,982
Gross Unrealized Depreciation	0	0
Allowance for Credit Losses	0	0
Foreign bonds
Fixed maturities:
Amortized Cost	26,372	27,799
Gross Unrealized Appreciation	1,171	1,805
Gross Unrealized Depreciation	134	2
Allowance for Credit Losses	0	0
Public utilities
Fixed maturities:
Amortized Cost	95,501	76,114
Gross Unrealized Appreciation	5,155	7,388
Gross Unrealized Depreciation	523	0
Allowance for Credit Losses	0	0
Corporate bonds | Energy
Fixed maturities:
Amortized Cost	24,101	22,441
Gross Unrealized Appreciation	2,306	2,895
Gross Unrealized Depreciation	0	0
Allowance for Credit Losses	0	0
Corporate bonds | Industrials
Fixed maturities:
Amortized Cost	53,535	39,513
Gross Unrealized Appreciation	2,618	3,744
Gross Unrealized Depreciation	572	0
Allowance for Credit Losses	0	0
Corporate bonds | Consumer goods and services
Fixed maturities:
Amortized Cost	66,557	46,521
Gross Unrealized Appreciation	3,233	4,046
Gross Unrealized Depreciation	1,285	0
Allowance for Credit Losses	0	0
Corporate bonds | Health care
Fixed maturities:
Amortized Cost	29,025	6,678
Gross Unrealized Appreciation	662	898
Gross Unrealized Depreciation	1,188	0
Allowance for Credit Losses	0	0
Corporate bonds | Technology, media and telecommunications
Fixed maturities:
Amortized Cost	66,211	37,270
Gross Unrealized Appreciation	3,679	4,381
Gross Unrealized Depreciation	1,956	15
Allowance for Credit Losses	0	0
Corporate bonds | Financial services
Fixed maturities:
Amortized Cost	101,002	93,736
Gross Unrealized Appreciation	5,504	7,564
Gross Unrealized Depreciation	487	269
Allowance for Credit Losses	174	5
Mortgage-backed securities
Fixed maturities:
Amortized Cost	26,092	20,305
Gross Unrealized Appreciation	351	326
Gross Unrealized Depreciation	143	54
Allowance for Credit Losses	0	0
Collateralized mortgage obligations | Government national mortgage association
Fixed maturities:
Amortized Cost	83,303	81,758
Gross Unrealized Appreciation	3,454	4,439
Gross Unrealized Depreciation	113	45
Allowance for Credit Losses	0	0
Collateralized mortgage obligations | Federal home loan mortgage corporation
Fixed maturities:
Amortized Cost	139,364	151,362
Gross Unrealized Appreciation	1,570	2,239
Gross Unrealized Depreciation	1,402	758
Allowance for Credit Losses	0	0
Collateralized mortgage obligations | Federal national mortgage association
Fixed maturities:
Amortized Cost	60,744	81,952
Gross Unrealized Appreciation	1,642	2,013
Gross Unrealized Depreciation	463	683
Allowance for Credit Losses	0	0
Asset-backed securities
Fixed maturities:
Amortized Cost	317	314
Gross Unrealized Appreciation	531	612
Gross Unrealized Depreciation	0	0
Allowance for Credit Losses	0	0
Fair Value
Fixed maturities:
Fair Value	1,817,743
Fair Value | Short-Term Investments
Fixed maturities:
Fair Value	804
Fair Value | Fixed maturities
Fixed maturities:
Fair Value	1,816,939	1,825,443
Fair Value | U.S. Treasury
Fixed maturities:
Fair Value	70,092	149,938
Fair Value | U.S. government agency
Fixed maturities:
Fair Value	88,157	64,518
Fair Value | States, municipalities and political subdivisions | General obligations | Midwest
Fixed maturities:
Fair Value	71,529	81,980
Fair Value | States, municipalities and political subdivisions | General obligations | Northeast
Fixed maturities:
Fair Value	30,099	30,450
Fair Value | States, municipalities and political subdivisions | General obligations | South
Fixed maturities:
Fair Value	99,188	109,970
Fair Value | States, municipalities and political subdivisions | General obligations | West
Fixed maturities:
Fair Value	103,501	110,021
Fair Value | States, municipalities and political subdivisions | Special revenue | Midwest
Fixed maturities:
Fair Value	131,658	125,098
Fair Value | States, municipalities and political subdivisions | Special revenue | Northeast
Fixed maturities:
Fair Value	60,165	61,076
Fair Value | States, municipalities and political subdivisions | Special revenue | South
Fixed maturities:
Fair Value	226,730	226,706
Fair Value | States, municipalities and political subdivisions | Special revenue | West
Fixed maturities:
Fair Value	140,086	139,399
Fair Value | Foreign bonds
Fixed maturities:
Fair Value	27,409	29,602
Fair Value | Public utilities
Fixed maturities:
Fair Value	100,133	83,502
Fair Value | Corporate bonds | Energy
Fixed maturities:
Fair Value	26,407	25,336
Fair Value | Corporate bonds | Industrials
Fixed maturities:
Fair Value	55,581	43,257
Fair Value | Corporate bonds | Consumer goods and services
Fixed maturities:
Fair Value	68,505	50,567
Fair Value | Corporate bonds | Health care
Fixed maturities:
Fair Value	28,499	7,576
Fair Value | Corporate bonds | Technology, media and telecommunications
Fixed maturities:
Fair Value	67,934	41,636
Fair Value | Corporate bonds | Financial services
Fixed maturities:
Fair Value	106,019	101,031
Fair Value | Mortgage-backed securities
Fixed maturities:
Fair Value	26,300	20,577
Fair Value | Collateralized mortgage obligations | Government national mortgage association
Fixed maturities:
Fair Value	86,644	86,152
Fair Value | Collateralized mortgage obligations | Federal home loan mortgage corporation
Fixed maturities:
Fair Value	139,532	152,843
Fair Value | Collateralized mortgage obligations | Federal national mortgage association
Fixed maturities:
Fair Value	61,923	83,282
Fair Value | Asset-backed securities
Fixed maturities:
Fair Value	848	926
Carrying Value
Fixed maturities:
Fair Value	1,817,569
Carrying Value | Short-Term Investments
Fixed maturities:
Fair Value	804
Carrying Value | Fixed maturities
Fixed maturities:
Fair Value	1,816,765	1,825,438
Carrying Value | U.S. Treasury
Fixed maturities:
Fair Value	70,092	149,938
Carrying Value | U.S. government agency
Fixed maturities:
Fair Value	88,157	64,518
Carrying Value | States, municipalities and political subdivisions | General obligations | Midwest
Fixed maturities:
Fair Value	71,529	81,980
Carrying Value | States, municipalities and political subdivisions | General obligations | Northeast
Fixed maturities:
Fair Value	30,099	30,450
Carrying Value | States, municipalities and political subdivisions | General obligations | South
Fixed maturities:
Fair Value	99,188	109,970
Carrying Value | States, municipalities and political subdivisions | General obligations | West
Fixed maturities:
Fair Value	103,501	110,021
Carrying Value | States, municipalities and political subdivisions | Special revenue | Midwest
Fixed maturities:
Fair Value	131,658	125,098
Carrying Value | States, municipalities and political subdivisions | Special revenue | Northeast
Fixed maturities:
Fair Value	60,165	61,076
Carrying Value | States, municipalities and political subdivisions | Special revenue | South
Fixed maturities:
Fair Value	226,730	226,706
Carrying Value | States, municipalities and political subdivisions | Special revenue | West
Fixed maturities:
Fair Value	140,086	139,399
Carrying Value | Foreign bonds
Fixed maturities:
Fair Value	27,409	29,602
Carrying Value | Public utilities
Fixed maturities:
Fair Value	100,133	83,502
Carrying Value | Corporate bonds | Energy
Fixed maturities:
Fair Value	26,407	25,336
Carrying Value | Corporate bonds | Industrials
Fixed maturities:
Fair Value	55,581	43,257
Carrying Value | Corporate bonds | Consumer goods and services
Fixed maturities:
Fair Value	68,505	50,567
Carrying Value | Corporate bonds | Health care
Fixed maturities:
Fair Value	28,499	7,576
Carrying Value | Corporate bonds | Technology, media and telecommunications
Fixed maturities:
Fair Value	67,934	41,636
Carrying Value | Corporate bonds | Financial services
Fixed maturities:
Fair Value	105,845	101,026
Carrying Value | Mortgage-backed securities
Fixed maturities:
Fair Value	26,300	20,577
Carrying Value | Collateralized mortgage obligations | Government national mortgage association
Fixed maturities:
Fair Value	86,644	86,152
Carrying Value | Collateralized mortgage obligations | Federal home loan mortgage corporation
Fixed maturities:
Fair Value	139,532	152,843
Carrying Value | Collateralized mortgage obligations | Federal national mortgage association
Fixed maturities:
Fair Value	61,923	83,282
Carrying Value | Asset-backed securities
Fixed maturities:
Fair Value	$ 848	$ 926