Summary of Investments (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fixed maturities:
Available-for-sale securities, amortized cost	$ 1,633,875	$ 1,659,760
Gross Unrealized Appreciation	69,153	60,639
Gross Unrealized Depreciation	3,610	792
Fair Value	1,699,359	1,719,607
Allowance for Credit Losses	59	0
U.S. Treasury
Fixed maturities:
Available-for-sale securities, amortized cost	49,068	69,300
Gross Unrealized Appreciation	1,026	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	75,504	97,962
Gross Unrealized Appreciation	4,184	2,344
Gross Unrealized Depreciation	0	104
Fair Value		100,202
Allowance for Credit Losses	0
Foreign bonds
Fixed maturities:
Available-for-sale securities, amortized cost	4,935	4,936
Gross Unrealized Appreciation	179	181
Gross Unrealized Depreciation	2	0
Fair Value		5,117
Allowance for Credit Losses	0
Public utilities
Fixed maturities:
Available-for-sale securities, amortized cost	74,192	60,950
Gross Unrealized Appreciation	2,072	2,701
Gross Unrealized Depreciation	424	0
Fair Value		63,651
Allowance for Credit Losses	0
Mortgage-backed securities
Fixed maturities:
Available-for-sale securities, amortized cost	5,929	6,250
Gross Unrealized Appreciation	205	127
Gross Unrealized Depreciation	4	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	76,808	78,400
Gross Unrealized Appreciation	5,334	2,053
Gross Unrealized Depreciation	30	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	114,099	123,572
Gross Unrealized Appreciation	4,494	1,150
Gross Unrealized Depreciation	7	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	76,401	70,322
Gross Unrealized Appreciation	4,000	1,631
Gross Unrealized Depreciation	26	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	607	436
Gross Unrealized Depreciation	0	0
Fair Value		750
Allowance for Credit Losses	0
General obligations | States, municipalities and political subdivisions | Midwest
Fixed maturities:
Available-for-sale securities, amortized cost	85,521	85,607
Gross Unrealized Appreciation	2,925	2,987
Gross Unrealized Depreciation	0	0
Fair Value		88,594
Allowance for Credit Losses	0
General obligations | States, municipalities and political subdivisions | Northeast
Fixed maturities:
Available-for-sale securities, amortized cost	30,067	30,120
Gross Unrealized Appreciation	1,071	1,150
Gross Unrealized Depreciation	0	0
Fair Value		31,270
Allowance for Credit Losses	0
General obligations | States, municipalities and political subdivisions | South
Fixed maturities:
Available-for-sale securities, amortized cost	111,497	111,688
Gross Unrealized Appreciation	3,579	3,515
Gross Unrealized Depreciation	0	0
Fair Value		115,203
Allowance for Credit Losses	0
General obligations | States, municipalities and political subdivisions | West
Fixed maturities:
Available-for-sale securities, amortized cost	105,363	105,569
Gross Unrealized Appreciation	4,712	4,748
Gross Unrealized Depreciation	0	0
Fair Value		110,317
Allowance for Credit Losses	0
Special revenue - south | States, municipalities and political subdivisions | Midwest
Fixed maturities:
Available-for-sale securities, amortized cost	130,758	133,717
Gross Unrealized Appreciation	6,740	6,175
Gross Unrealized Depreciation	0	0
Fair Value		139,892
Allowance for Credit Losses	0
Special revenue - south | States, municipalities and political subdivisions | Northeast
Fixed maturities:
Available-for-sale securities, amortized cost	58,540	58,665
Gross Unrealized Appreciation	2,892	2,878
Gross Unrealized Depreciation	0	0
Fair Value		61,543
Allowance for Credit Losses	0
Special revenue - south | States, municipalities and political subdivisions | South
Fixed maturities:
Available-for-sale securities, amortized cost	223,360	224,214
Gross Unrealized Appreciation	10,617	10,452
Gross Unrealized Depreciation	48	0
Fair Value		234,666
Allowance for Credit Losses	0
Special revenue - south | States, municipalities and political subdivisions | West
Fixed maturities:
Available-for-sale securities, amortized cost	137,256	138,557
Gross Unrealized Appreciation	6,051	6,287
Gross Unrealized Depreciation	0	0
Fair Value		144,844
Allowance for Credit Losses	0
Energy | Corporate bonds
Fixed maturities:
Available-for-sale securities, amortized cost	27,124	28,695
Gross Unrealized Appreciation	236	1,429
Gross Unrealized Depreciation	1,360	0
Fair Value		30,124
Allowance for Credit Losses	16
Industrials | Corporate bonds
Fixed maturities:
Available-for-sale securities, amortized cost	56,173	52,249
Gross Unrealized Appreciation	1,640	1,766
Gross Unrealized Depreciation	380	0
Fair Value		54,015
Allowance for Credit Losses	0
Consumer goods and services | Corporate bonds
Fixed maturities:
Available-for-sale securities, amortized cost	47,109	47,131
Gross Unrealized Appreciation	1,925	2,335
Gross Unrealized Depreciation	263	0
Fair Value		49,466
Allowance for Credit Losses	0
Health care | Corporate bonds
Fixed maturities:
Available-for-sale securities, amortized cost	9,006	8,998
Gross Unrealized Appreciation	368	482
Gross Unrealized Depreciation	0	0
Fair Value		9,480
Allowance for Credit Losses	0
Technology, media and telecommunications | Corporate bonds
Fixed maturities:
Available-for-sale securities, amortized cost	34,185	25,931
Gross Unrealized Appreciation	1,618	1,739
Gross Unrealized Depreciation	272	0
Fair Value		27,670
Allowance for Credit Losses	11
Financial services | Corporate bonds
Fixed maturities:
Available-for-sale securities, amortized cost	100,666	96,613
Gross Unrealized Appreciation	2,678	3,870
Gross Unrealized Depreciation	794	230
Fair Value		100,253
Allowance for Credit Losses	32
Fair Value
Fixed maturities:
Fair Value	1,699,418	1,719,607
Fair Value | U.S. Treasury
Fixed maturities:
Fair Value	50,094
Fair Value | U.S. government agency
Fixed maturities:
Fair Value	79,688
Fair Value | Foreign bonds
Fixed maturities:
Fair Value	5,112
Fair Value | Public utilities
Fixed maturities:
Fair Value	75,840
Fair Value | Mortgage-backed securities
Fixed maturities:
Fair Value	6,130
Fair Value | Collateralized mortgage obligations | Government national mortgage association
Fixed maturities:
Fair Value	82,112
Fair Value | Collateralized mortgage obligations | Federal home loan mortgage corporation
Fixed maturities:
Fair Value	118,586
Fair Value | Collateralized mortgage obligations | Federal national mortgage association
Fixed maturities:
Fair Value	80,375
Fair Value | Asset-backed securities
Fixed maturities:
Fair Value	921
Fair Value | General obligations | States, municipalities and political subdivisions | Midwest
Fixed maturities:
Fair Value	88,446
Fair Value | General obligations | States, municipalities and political subdivisions | Northeast
Fixed maturities:
Fair Value	31,138
Fair Value | General obligations | States, municipalities and political subdivisions | South
Fixed maturities:
Fair Value	115,076
Fair Value | General obligations | States, municipalities and political subdivisions | West
Fixed maturities:
Fair Value	110,075
Fair Value | Special revenue - south | States, municipalities and political subdivisions | Midwest
Fixed maturities:
Fair Value	137,498
Fair Value | Special revenue - south | States, municipalities and political subdivisions | Northeast
Fixed maturities:
Fair Value	61,432
Fair Value | Special revenue - south | States, municipalities and political subdivisions | South
Fixed maturities:
Fair Value	233,929
Fair Value | Special revenue - south | States, municipalities and political subdivisions | West
Fixed maturities:
Fair Value	143,307
Fair Value | Energy | Corporate bonds
Fixed maturities:
Fair Value	26,000
Fair Value | Industrials | Corporate bonds
Fixed maturities:
Fair Value	57,433
Fair Value | Consumer goods and services | Corporate bonds
Fixed maturities:
Fair Value	48,771
Fair Value | Health care | Corporate bonds
Fixed maturities:
Fair Value	9,374
Fair Value | Technology, media and telecommunications | Corporate bonds
Fixed maturities:
Fair Value	35,531
Fair Value | Financial services | Corporate bonds
Fixed maturities:
Fair Value	102,550
Carrying Value
Fixed maturities:
Fair Value	1,699,359	$ 1,719,607
Carrying Value | U.S. Treasury
Fixed maturities:
Fair Value	50,094
Carrying Value | U.S. government agency
Fixed maturities:
Fair Value	79,688
Carrying Value | Foreign bonds
Fixed maturities:
Fair Value	5,112
Carrying Value | Public utilities
Fixed maturities:
Fair Value	75,840
Carrying Value | Mortgage-backed securities
Fixed maturities:
Fair Value	6,130
Carrying Value | Collateralized mortgage obligations | Government national mortgage association
Fixed maturities:
Fair Value	82,112
Carrying Value | Collateralized mortgage obligations | Federal home loan mortgage corporation
Fixed maturities:
Fair Value	118,586
Carrying Value | Collateralized mortgage obligations | Federal national mortgage association
Fixed maturities:
Fair Value	80,375
Carrying Value | Asset-backed securities
Fixed maturities:
Fair Value	921
Carrying Value | General obligations | States, municipalities and political subdivisions | Midwest
Fixed maturities:
Fair Value	88,446
Carrying Value | General obligations | States, municipalities and political subdivisions | Northeast
Fixed maturities:
Fair Value	31,138
Carrying Value | General obligations | States, municipalities and political subdivisions | South
Fixed maturities:
Fair Value	115,076
Carrying Value | General obligations | States, municipalities and political subdivisions | West
Fixed maturities:
Fair Value	110,075
Carrying Value | Special revenue - south | States, municipalities and political subdivisions | Midwest
Fixed maturities:
Fair Value	137,498
Carrying Value | Special revenue - south | States, municipalities and political subdivisions | Northeast
Fixed maturities:
Fair Value	61,432
Carrying Value | Special revenue - south | States, municipalities and political subdivisions | South
Fixed maturities:
Fair Value	233,929
Carrying Value | Special revenue - south | States, municipalities and political subdivisions | West
Fixed maturities:
Fair Value	143,307
Carrying Value | Energy | Corporate bonds
Fixed maturities:
Fair Value	25,984
Carrying Value | Industrials | Corporate bonds
Fixed maturities:
Fair Value	57,433
Carrying Value | Consumer goods and services | Corporate bonds
Fixed maturities:
Fair Value	48,771
Carrying Value | Health care | Corporate bonds
Fixed maturities:
Fair Value	9,374
Carrying Value | Technology, media and telecommunications | Corporate bonds
Fixed maturities:
Fair Value	35,520
Carrying Value | Financial services | Corporate bonds
Fixed maturities:
Fair Value	$ 102,518