Summary of Investments (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fixed maturities:
Amortized Cost	$ 1,709,100	$ 1,761,289
Gross Unrealized Appreciation	28,540	11,844
Gross Unrealized Depreciation	7,077	23,645
Fair Value	1,730,563	1,749,488
U.S. Treasury
Fixed maturities:
Amortized Cost	26,821	27,632
Gross Unrealized Appreciation	13	6
Gross Unrealized Depreciation	143	220
Fair Value	26,691	27,418
U.S. government agency
Fixed maturities:
Amortized Cost	188,529	215,535
Gross Unrealized Appreciation	1,417	896
Gross Unrealized Depreciation	405	1,749
Fair Value	189,541	214,682
States, municipalities and political subdivisions | General obligations | Midwest
Fixed maturities:
Amortized Cost	86,782	94,806
Gross Unrealized Appreciation	1,995	1,091
Gross Unrealized Depreciation	98	685
Fair Value	88,679	95,212
States, municipalities and political subdivisions | General obligations | Northeast
Fixed maturities:
Amortized Cost	31,266	37,326
Gross Unrealized Appreciation	866	432
Gross Unrealized Depreciation	0	103
Fair Value	32,132	37,655
States, municipalities and political subdivisions | General obligations | South
Fixed maturities:
Amortized Cost	114,500	114,710
Gross Unrealized Appreciation	1,755	754
Gross Unrealized Depreciation	475	1,553
Fair Value	115,780	113,911
States, municipalities and political subdivisions | General obligations | West
Fixed maturities:
Amortized Cost	107,580	107,787
Gross Unrealized Appreciation	2,592	1,229
Gross Unrealized Depreciation	305	1,175
Fair Value	109,867	107,841
States, municipalities and political subdivisions | Special revenue | Midwest
Fixed maturities:
Amortized Cost	139,803	140,025
Gross Unrealized Appreciation	3,520	1,609
Gross Unrealized Depreciation	194	870
Fair Value	143,129	140,764
States, municipalities and political subdivisions | Special revenue | Northeast
Fixed maturities:
Amortized Cost	62,604	62,737
Gross Unrealized Appreciation	1,123	452
Gross Unrealized Depreciation	406	1,241
Fair Value	63,321	61,948
States, municipalities and political subdivisions | Special revenue | South
Fixed maturities:
Amortized Cost	229,505	237,848
Gross Unrealized Appreciation	4,532	1,669
Gross Unrealized Depreciation	1,101	3,708
Fair Value	232,936	235,809
States, municipalities and political subdivisions | Special revenue | West
Fixed maturities:
Amortized Cost	142,974	143,829
Gross Unrealized Appreciation	2,905	1,294
Gross Unrealized Depreciation	537	2,203
Fair Value	145,342	142,920
Foreign bonds
Fixed maturities:
Amortized Cost	4,941	9,698
Gross Unrealized Appreciation	106	31
Gross Unrealized Depreciation	0	13
Fair Value	5,047	9,716
Public utilities
Fixed maturities:
Amortized Cost	62,774	56,808
Gross Unrealized Appreciation	878	274
Gross Unrealized Depreciation	444	1,023
Fair Value	63,208	56,059
Corporate bonds | Energy
Fixed maturities:
Amortized Cost	28,158	28,909
Gross Unrealized Appreciation	536	43
Gross Unrealized Depreciation	41	304
Fair Value	28,653	28,648
Corporate bonds | Industrials
Fixed maturities:
Amortized Cost	53,836	53,867
Gross Unrealized Appreciation	613	124
Gross Unrealized Depreciation	171	906
Fair Value	54,278	53,085
Corporate bonds | Consumer goods and services
Fixed maturities:
Amortized Cost	48,151	54,323
Gross Unrealized Appreciation	857	142
Gross Unrealized Depreciation	209	819
Fair Value	48,799	53,646
Corporate bonds | Health care
Fixed maturities:
Amortized Cost	13,974	16,721
Gross Unrealized Appreciation	294	42
Gross Unrealized Depreciation	2	105
Fair Value	14,266	16,658
Corporate bonds | Technology, media and telecommunications
Fixed maturities:
Amortized Cost	25,895	26,819
Gross Unrealized Appreciation	463	35
Gross Unrealized Depreciation	138	678
Fair Value	26,220	26,176
Corporate bonds | Financial services
Fixed maturities:
Amortized Cost	91,876	81,286
Gross Unrealized Appreciation	1,366	238
Gross Unrealized Depreciation	669	2,175
Fair Value	92,573	79,349
Mortgage-backed securities
Fixed maturities:
Amortized Cost	7,401	7,642
Gross Unrealized Appreciation	48	14
Gross Unrealized Depreciation	134	232
Fair Value	7,315	7,424
Asset-backed securities
Fixed maturities:
Amortized Cost	3,274	3,256
Gross Unrealized Appreciation	387	352
Gross Unrealized Depreciation	125	113
Fair Value	3,536	3,495
Government National Mortgage Association (GNMA) Insured Loans | Collateralized mortgage obligations
Fixed maturities:
Amortized Cost	77,401	78,055
Gross Unrealized Appreciation	750	380
Gross Unrealized Depreciation	763	1,734
Fair Value	77,388	76,701
Federal Home Loan Mortgage Corporation (FHLMC) Insured Loans | Collateralized mortgage obligations
Fixed maturities:
Amortized Cost	108,333	108,403
Gross Unrealized Appreciation	889	524
Gross Unrealized Depreciation	513	1,304
Fair Value	108,709	107,623
Federal National Mortgage Association (FNMA) Insured Loans | Collateralized mortgage obligations
Fixed maturities:
Amortized Cost	52,722	53,267
Gross Unrealized Appreciation	635	213
Gross Unrealized Depreciation	204	732
Fair Value	$ 53,153	$ 52,748