Summary of Investments (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fixed maturities:
Cost or Amortized Cost	$ 1,612,626	$ 1,659,760
Gross Unrealized Appreciation	105,803	60,639
Gross Unrealized Depreciation	1,382	792
Fair Value	1,717,039	1,719,607
Allowance for Credit Losses	8	0
U.S. Treasury
Fixed maturities:
Cost or Amortized Cost	53,877	69,300
Gross Unrealized Appreciation	730	203
Gross Unrealized Depreciation	8	12
Fair Value		69,491
Allowance for Credit Losses	0
U.S. government agency
Fixed maturities:
Cost or Amortized Cost	60,503	97,962
Gross Unrealized Appreciation	4,457	2,344
Gross Unrealized Depreciation	0	104
Fair Value		100,202
Allowance for Credit Losses	0
States, municipalities and political subdivisions | General obligations | Midwest
Fixed maturities:
Cost or Amortized Cost	79,872	85,607
Gross Unrealized Appreciation	4,106	2,987
Gross Unrealized Depreciation	0	0
Fair Value		88,594
Allowance for Credit Losses	0
States, municipalities and political subdivisions | General obligations | Northeast
Fixed maturities:
Cost or Amortized Cost	29,124	30,120
Gross Unrealized Appreciation	1,426	1,150
Gross Unrealized Depreciation	0	0
Fair Value		31,270
Allowance for Credit Losses	0
States, municipalities and political subdivisions | General obligations | South
Fixed maturities:
Cost or Amortized Cost	105,290	111,688
Gross Unrealized Appreciation	5,295	3,515
Gross Unrealized Depreciation	0	0
Fair Value		115,203
Allowance for Credit Losses	0
States, municipalities and political subdivisions | General obligations | West
Fixed maturities:
Cost or Amortized Cost	102,772	105,569
Gross Unrealized Appreciation	7,175	4,748
Gross Unrealized Depreciation	0	0
Fair Value		110,317
Allowance for Credit Losses	0
States, municipalities and political subdivisions | Special revenue | Midwest
Fixed maturities:
Cost or Amortized Cost	126,251	133,717
Gross Unrealized Appreciation	8,590	6,175
Gross Unrealized Depreciation	0	0
Fair Value		139,892
Allowance for Credit Losses	0
States, municipalities and political subdivisions | Special revenue | Northeast
Fixed maturities:
Cost or Amortized Cost	58,287	58,665
Gross Unrealized Appreciation	4,557	2,878
Gross Unrealized Depreciation	0	0
Fair Value		61,543
Allowance for Credit Losses	0
States, municipalities and political subdivisions | Special revenue | South
Fixed maturities:
Cost or Amortized Cost	220,904	224,214
Gross Unrealized Appreciation	17,446	10,452
Gross Unrealized Depreciation	0	0
Fair Value		234,666
Allowance for Credit Losses	0
States, municipalities and political subdivisions | Special revenue | West
Fixed maturities:
Cost or Amortized Cost	133,934	138,557
Gross Unrealized Appreciation	9,168	6,287
Gross Unrealized Depreciation	0	0
Fair Value		144,844
Allowance for Credit Losses	0
Foreign bonds
Fixed maturities:
Cost or Amortized Cost	26,413	4,936
Gross Unrealized Appreciation	1,835	181
Gross Unrealized Depreciation	230	0
Fair Value		5,117
Allowance for Credit Losses	0
Public utilities
Fixed maturities:
Cost or Amortized Cost	77,147	60,950
Gross Unrealized Appreciation	7,155	2,701
Gross Unrealized Depreciation	0	0
Fair Value		63,651
Allowance for Credit Losses	0
Corporate bonds | Energy
Fixed maturities:
Cost or Amortized Cost	24,912	28,695
Gross Unrealized Appreciation	2,589	1,429
Gross Unrealized Depreciation	0	0
Fair Value		30,124
Allowance for Credit Losses	0
Corporate bonds | Industrials
Fixed maturities:
Cost or Amortized Cost	39,558	52,249
Gross Unrealized Appreciation	3,499	1,766
Gross Unrealized Depreciation	0	0
Fair Value		54,015
Allowance for Credit Losses	0
Corporate bonds | Consumer goods and services
Fixed maturities:
Cost or Amortized Cost	45,250	47,131
Gross Unrealized Appreciation	3,790	2,335
Gross Unrealized Depreciation	12	0
Fair Value		49,466
Allowance for Credit Losses	0
Corporate bonds | Health care
Fixed maturities:
Cost or Amortized Cost	6,674	8,998
Gross Unrealized Appreciation	895	482
Gross Unrealized Depreciation	0	0
Fair Value		9,480
Allowance for Credit Losses	0
Corporate bonds | Technology, media and telecommunications
Fixed maturities:
Cost or Amortized Cost	37,241	25,931
Gross Unrealized Appreciation	4,330	1,739
Gross Unrealized Depreciation	0	0
Fair Value		27,670
Allowance for Credit Losses	0
Corporate bonds | Financial services
Fixed maturities:
Cost or Amortized Cost	95,696	96,613
Gross Unrealized Appreciation	7,030	3,870
Gross Unrealized Depreciation	267	230
Fair Value		100,253
Allowance for Credit Losses	8
Mortgage-backed securities
Fixed maturities:
Cost or Amortized Cost	14,333	6,250
Gross Unrealized Appreciation	305	127
Gross Unrealized Depreciation	109	21
Fair Value		6,356
Allowance for Credit Losses	0
Collateralized mortgage obligations | Government national mortgage association
Fixed maturities:
Cost or Amortized Cost	71,992	78,400
Gross Unrealized Appreciation	5,206	2,053
Gross Unrealized Depreciation	1	97
Fair Value		80,356
Allowance for Credit Losses	0
Collateralized mortgage obligations | Federal home loan mortgage corporation
Fixed maturities:
Cost or Amortized Cost	119,006	123,572
Gross Unrealized Appreciation	3,019	1,150
Gross Unrealized Depreciation	349	220
Fair Value		124,502
Allowance for Credit Losses	0
Collateralized mortgage obligations | Federal national mortgage association
Fixed maturities:
Cost or Amortized Cost	83,276	70,322
Gross Unrealized Appreciation	2,579	1,631
Gross Unrealized Depreciation	406	108
Fair Value		71,845
Allowance for Credit Losses	0
Asset-backed securities
Fixed maturities:
Cost or Amortized Cost	314	314
Gross Unrealized Appreciation	621	436
Gross Unrealized Depreciation	0	0
Fair Value		750
Allowance for Credit Losses	0
Fair Value
Fixed maturities:
Fair Value	1,717,047	1,719,607
Fair Value | U.S. Treasury
Fixed maturities:
Fair Value	54,599
Fair Value | U.S. government agency
Fixed maturities:
Fair Value	64,960
Fair Value | States, municipalities and political subdivisions | General obligations | Midwest
Fixed maturities:
Fair Value	83,978
Fair Value | States, municipalities and political subdivisions | General obligations | Northeast
Fixed maturities:
Fair Value	30,550
Fair Value | States, municipalities and political subdivisions | General obligations | South
Fixed maturities:
Fair Value	110,585
Fair Value | States, municipalities and political subdivisions | General obligations | West
Fixed maturities:
Fair Value	109,947
Fair Value | States, municipalities and political subdivisions | Special revenue | Midwest
Fixed maturities:
Fair Value	134,841
Fair Value | States, municipalities and political subdivisions | Special revenue | Northeast
Fixed maturities:
Fair Value	62,844
Fair Value | States, municipalities and political subdivisions | Special revenue | South
Fixed maturities:
Fair Value	238,350
Fair Value | States, municipalities and political subdivisions | Special revenue | West
Fixed maturities:
Fair Value	143,102
Fair Value | Foreign bonds
Fixed maturities:
Fair Value	28,018
Fair Value | Public utilities
Fixed maturities:
Fair Value	84,302
Fair Value | Corporate bonds | Energy
Fixed maturities:
Fair Value	27,501
Fair Value | Corporate bonds | Industrials
Fixed maturities:
Fair Value	43,057
Fair Value | Corporate bonds | Consumer goods and services
Fixed maturities:
Fair Value	49,028
Fair Value | Corporate bonds | Health care
Fixed maturities:
Fair Value	7,569
Fair Value | Corporate bonds | Technology, media and telecommunications
Fixed maturities:
Fair Value	41,571
Fair Value | Corporate bonds | Financial services
Fixed maturities:
Fair Value	102,459
Fair Value | Mortgage-backed securities
Fixed maturities:
Fair Value	14,529
Fair Value | Collateralized mortgage obligations | Government national mortgage association
Fixed maturities:
Fair Value	77,197
Fair Value | Collateralized mortgage obligations | Federal home loan mortgage corporation
Fixed maturities:
Fair Value	121,676
Fair Value | Collateralized mortgage obligations | Federal national mortgage association
Fixed maturities:
Fair Value	85,449
Fair Value | Asset-backed securities
Fixed maturities:
Fair Value	935
Carrying Value
Fixed maturities:
Fair Value	1,717,039	$ 1,719,607
Carrying Value | U.S. Treasury
Fixed maturities:
Fair Value	54,599
Carrying Value | U.S. government agency
Fixed maturities:
Fair Value	64,960
Carrying Value | States, municipalities and political subdivisions | General obligations | Midwest
Fixed maturities:
Fair Value	83,978
Carrying Value | States, municipalities and political subdivisions | General obligations | Northeast
Fixed maturities:
Fair Value	30,550
Carrying Value | States, municipalities and political subdivisions | General obligations | South
Fixed maturities:
Fair Value	110,585
Carrying Value | States, municipalities and political subdivisions | General obligations | West
Fixed maturities:
Fair Value	109,947
Carrying Value | States, municipalities and political subdivisions | Special revenue | Midwest
Fixed maturities:
Fair Value	134,841
Carrying Value | States, municipalities and political subdivisions | Special revenue | Northeast
Fixed maturities:
Fair Value	62,844
Carrying Value | States, municipalities and political subdivisions | Special revenue | South
Fixed maturities:
Fair Value	238,350
Carrying Value | States, municipalities and political subdivisions | Special revenue | West
Fixed maturities:
Fair Value	143,102
Carrying Value | Foreign bonds
Fixed maturities:
Fair Value	28,018
Carrying Value | Public utilities
Fixed maturities:
Fair Value	84,302
Carrying Value | Corporate bonds | Energy
Fixed maturities:
Fair Value	27,501
Carrying Value | Corporate bonds | Industrials
Fixed maturities:
Fair Value	43,057
Carrying Value | Corporate bonds | Consumer goods and services
Fixed maturities:
Fair Value	49,028
Carrying Value | Corporate bonds | Health care
Fixed maturities:
Fair Value	7,569
Carrying Value | Corporate bonds | Technology, media and telecommunications
Fixed maturities:
Fair Value	41,571
Carrying Value | Corporate bonds | Financial services
Fixed maturities:
Fair Value	102,451
Carrying Value | Mortgage-backed securities
Fixed maturities:
Fair Value	14,529
Carrying Value | Collateralized mortgage obligations | Government national mortgage association
Fixed maturities:
Fair Value	77,197
Carrying Value | Collateralized mortgage obligations | Federal home loan mortgage corporation
Fixed maturities:
Fair Value	121,676
Carrying Value | Collateralized mortgage obligations | Federal national mortgage association
Fixed maturities:
Fair Value	85,449
Carrying Value | Asset-backed securities
Fixed maturities:
Fair Value	$ 935