Summary of Investments (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fixed maturities:
Available-for-sale securities, amortized cost	$ 1,595,817	$ 1,659,760
Gross Unrealized Appreciation	101,091	60,639
Gross Unrealized Depreciation	732	792
Fair Value	1,696,166	1,719,607
Allowance for Credit Losses	10	0
U.S. Treasury
Fixed maturities:
Available-for-sale securities, amortized cost	50,353	69,300
Gross Unrealized Appreciation	869	203
Gross Unrealized Depreciation	0	12
Fair Value		69,491
Allowance for Credit Losses	0
U.S. government agency
Fixed maturities:
Available-for-sale securities, amortized cost	63,503	97,962
Gross Unrealized Appreciation	4,005	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:
Available-for-sale securities, amortized cost	81,486	85,607
Gross Unrealized Appreciation	4,068	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:
Available-for-sale securities, amortized cost	30,012	30,120
Gross Unrealized Appreciation	1,487	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:
Available-for-sale securities, amortized cost	105,479	111,688
Gross Unrealized Appreciation	5,126	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:
Available-for-sale securities, amortized cost	105,196	105,569
Gross Unrealized Appreciation	6,647	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:
Available-for-sale securities, amortized cost	127,824	133,717
Gross Unrealized Appreciation	8,508	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:
Available-for-sale securities, amortized cost	58,414	58,665
Gross Unrealized Appreciation	4,218	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:
Available-for-sale securities, amortized cost	221,309	224,214
Gross Unrealized Appreciation	15,894	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:
Available-for-sale securities, amortized cost	134,462	138,557
Gross Unrealized Appreciation	8,532	6,287
Gross Unrealized Depreciation	0	0
Fair Value		144,844
Allowance for Credit Losses	0
Foreign bonds
Fixed maturities:
Available-for-sale securities, amortized cost	26,428	4,936
Gross Unrealized Appreciation	1,739	181
Gross Unrealized Depreciation	106	0
Fair Value		5,117
Allowance for Credit Losses	0
Public utilities
Fixed maturities:
Available-for-sale securities, amortized cost	77,529	60,950
Gross Unrealized Appreciation	6,713	2,701
Gross Unrealized Depreciation	0	0
Fair Value		63,651
Allowance for Credit Losses	0
Corporate bonds | Energy
Fixed maturities:
Available-for-sale securities, amortized cost	25,928	28,695
Gross Unrealized Appreciation	2,353	1,429
Gross Unrealized Depreciation	11	0
Fair Value		30,124
Allowance for Credit Losses	0
Corporate bonds | Industrials
Fixed maturities:
Available-for-sale securities, amortized cost	39,625	52,249
Gross Unrealized Appreciation	3,115	1,766
Gross Unrealized Depreciation	0	0
Fair Value		54,015
Allowance for Credit Losses	0
Corporate bonds | Consumer goods and services
Fixed maturities:
Available-for-sale securities, amortized cost	47,134	47,131
Gross Unrealized Appreciation	3,726	2,335
Gross Unrealized Depreciation	52	0
Fair Value		49,466
Allowance for Credit Losses	0
Corporate bonds | Health care
Fixed maturities:
Available-for-sale securities, amortized cost	7,327	8,998
Gross Unrealized Appreciation	869	482
Gross Unrealized Depreciation	0	0
Fair Value		9,480
Allowance for Credit Losses	0
Corporate bonds | Technology, media and telecommunications
Fixed maturities:
Available-for-sale securities, amortized cost	35,253	25,931
Gross Unrealized Appreciation	4,010	1,739
Gross Unrealized Depreciation	0	0
Fair Value		27,670
Allowance for Credit Losses	0
Corporate bonds | Financial services
Fixed maturities:
Available-for-sale securities, amortized cost	96,415	96,613
Gross Unrealized Appreciation	6,680	3,870
Gross Unrealized Depreciation	317	230
Fair Value		100,253
Allowance for Credit Losses	10
Mortgage-backed securities
Fixed maturities:
Available-for-sale securities, amortized cost	15,737	6,250
Gross Unrealized Appreciation	335	127
Gross Unrealized Depreciation	2	21
Fair Value		6,356
Allowance for Credit Losses	0
Collateralized mortgage obligations | Government national mortgage association
Fixed maturities:
Available-for-sale securities, amortized cost	74,741	78,400
Gross Unrealized Appreciation	5,707	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:
Available-for-sale securities, amortized cost	95,076	123,572
Gross Unrealized Appreciation	2,862	1,150
Gross Unrealized Depreciation	160	220
Fair Value		124,502
Allowance for Credit Losses	0
Collateralized mortgage obligations | Federal national mortgage association
Fixed maturities:
Available-for-sale securities, amortized cost	76,272	70,322
Gross Unrealized Appreciation	3,015	1,631
Gross Unrealized Depreciation	83	108
Fair Value		71,845
Allowance for Credit Losses	0
Asset-backed securities
Fixed maturities:
Available-for-sale securities, amortized cost	314	314
Gross Unrealized Appreciation	613	436
Gross Unrealized Depreciation	0	0
Fair Value		750
Allowance for Credit Losses	0
Fair Value
Fixed maturities:
Fair Value	1,696,176	1,719,607
Fair Value | U.S. Treasury
Fixed maturities:
Fair Value	51,222
Fair Value | U.S. government agency
Fixed maturities:
Fair Value	67,508
Fair Value | States, municipalities and political subdivisions | General obligations | Midwest
Fixed maturities:
Fair Value	85,554
Fair Value | States, municipalities and political subdivisions | General obligations | Northeast
Fixed maturities:
Fair Value	31,499
Fair Value | States, municipalities and political subdivisions | General obligations | South
Fixed maturities:
Fair Value	110,605
Fair Value | States, municipalities and political subdivisions | General obligations | West
Fixed maturities:
Fair Value	111,843
Fair Value | States, municipalities and political subdivisions | Special revenue | Midwest
Fixed maturities:
Fair Value	136,332
Fair Value | States, municipalities and political subdivisions | Special revenue | Northeast
Fixed maturities:
Fair Value	62,632
Fair Value | States, municipalities and political subdivisions | Special revenue | South
Fixed maturities:
Fair Value	237,203
Fair Value | States, municipalities and political subdivisions | Special revenue | West
Fixed maturities:
Fair Value	142,994
Fair Value | Foreign bonds
Fixed maturities:
Fair Value	28,061
Fair Value | Public utilities
Fixed maturities:
Fair Value	84,242
Fair Value | Corporate bonds | Energy
Fixed maturities:
Fair Value	28,270
Fair Value | Corporate bonds | Industrials
Fixed maturities:
Fair Value	42,740
Fair Value | Corporate bonds | Consumer goods and services
Fixed maturities:
Fair Value	50,808
Fair Value | Corporate bonds | Health care
Fixed maturities:
Fair Value	8,196
Fair Value | Corporate bonds | Technology, media and telecommunications
Fixed maturities:
Fair Value	39,263
Fair Value | Corporate bonds | Financial services
Fixed maturities:
Fair Value	102,778
Fair Value | Mortgage-backed securities
Fixed maturities:
Fair Value	16,070
Fair Value | Collateralized mortgage obligations | Government national mortgage association
Fixed maturities:
Fair Value	80,447
Fair Value | Collateralized mortgage obligations | Federal home loan mortgage corporation
Fixed maturities:
Fair Value	97,778
Fair Value | Collateralized mortgage obligations | Federal national mortgage association
Fixed maturities:
Fair Value	79,204
Fair Value | Asset-backed securities
Fixed maturities:
Fair Value	927
Carrying Value
Fixed maturities:
Fair Value	1,696,166	$ 1,719,607
Carrying Value | U.S. Treasury
Fixed maturities:
Fair Value	51,222
Carrying Value | U.S. government agency
Fixed maturities:
Fair Value	67,508
Carrying Value | States, municipalities and political subdivisions | General obligations | Midwest
Fixed maturities:
Fair Value	85,554
Carrying Value | States, municipalities and political subdivisions | General obligations | Northeast
Fixed maturities:
Fair Value	31,499
Carrying Value | States, municipalities and political subdivisions | General obligations | South
Fixed maturities:
Fair Value	110,605
Carrying Value | States, municipalities and political subdivisions | General obligations | West
Fixed maturities:
Fair Value	111,843
Carrying Value | States, municipalities and political subdivisions | Special revenue | Midwest
Fixed maturities:
Fair Value	136,332
Carrying Value | States, municipalities and political subdivisions | Special revenue | Northeast
Fixed maturities:
Fair Value	62,632
Carrying Value | States, municipalities and political subdivisions | Special revenue | South
Fixed maturities:
Fair Value	237,203
Carrying Value | States, municipalities and political subdivisions | Special revenue | West
Fixed maturities:
Fair Value	142,994
Carrying Value | Foreign bonds
Fixed maturities:
Fair Value	28,061
Carrying Value | Public utilities
Fixed maturities:
Fair Value	84,242
Carrying Value | Corporate bonds | Energy
Fixed maturities:
Fair Value	28,270
Carrying Value | Corporate bonds | Industrials
Fixed maturities:
Fair Value	42,740
Carrying Value | Corporate bonds | Consumer goods and services
Fixed maturities:
Fair Value	50,808
Carrying Value | Corporate bonds | Health care
Fixed maturities:
Fair Value	8,196
Carrying Value | Corporate bonds | Technology, media and telecommunications
Fixed maturities:
Fair Value	39,263
Carrying Value | Corporate bonds | Financial services
Fixed maturities:
Fair Value	102,768
Carrying Value | Mortgage-backed securities
Fixed maturities:
Fair Value	16,070
Carrying Value | Collateralized mortgage obligations | Government national mortgage association
Fixed maturities:
Fair Value	80,447
Carrying Value | Collateralized mortgage obligations | Federal home loan mortgage corporation
Fixed maturities:
Fair Value	97,778
Carrying Value | Collateralized mortgage obligations | Federal national mortgage association
Fixed maturities:
Fair Value	79,204
Carrying Value | Asset-backed securities
Fixed maturities:
Fair Value	$ 927