Summary of Investments (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
HELD-TO-MATURITY
Cost or Amortized Cost	$ 195	$ 198
Fair Value	196	199
AVAILABLE-FOR-SALE
Cost or Amortized Cost	2,982,501	2,956,009
Gross Unrealized Appreciation	255,596	247,298
Gross Unrealized Depreciation	30,225	34,765
Fair Value	3,207,872	3,168,542
Fixed Maturities
HELD-TO-MATURITY
Cost or Amortized Cost	195	198
Gross Unrealized Appreciation	1	1
Gross Unrealized Depreciation	0	0
Fair Value	196	199
AVAILABLE-FOR-SALE
Cost or Amortized Cost	2,914,524	2,887,505
Gross Unrealized Appreciation	47,387	44,840
Gross Unrealized Depreciation	29,723	34,219
Fair Value	2,932,188	2,898,126
Corporate bonds | Energy
AVAILABLE-FOR-SALE
Cost or Amortized Cost	103,194	107,084
Gross Unrealized Appreciation	2,190	2,195
Gross Unrealized Depreciation	193	419
Fair Value	105,191	108,860
Corporate bonds | Industrials
AVAILABLE-FOR-SALE
Cost or Amortized Cost	222,476	225,526
Gross Unrealized Appreciation	5,017	5,359
Gross Unrealized Depreciation	498	982
Fair Value	226,995	229,903
Corporate bonds | Consumer goods and services
AVAILABLE-FOR-SALE
Cost or Amortized Cost	176,581	178,135
Gross Unrealized Appreciation	4,089	3,847
Gross Unrealized Depreciation	269	295
Fair Value	180,401	181,687
Corporate bonds | Health care
AVAILABLE-FOR-SALE
Cost or Amortized Cost	75,951	81,211
Gross Unrealized Appreciation	2,039	2,063
Gross Unrealized Depreciation	37	151
Fair Value	77,953	83,123
Corporate bonds | Technology, media and telecommunications
AVAILABLE-FOR-SALE
Cost or Amortized Cost	143,113	143,402
Gross Unrealized Appreciation	2,562	2,029
Gross Unrealized Depreciation	700	819
Fair Value	144,975	144,612
Corporate bonds | Financial services
HELD-TO-MATURITY
Cost or Amortized Cost	150	150
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	0	0
Fair Value	150	150
AVAILABLE-FOR-SALE
Cost or Amortized Cost	272,088	269,981
Gross Unrealized Appreciation	5,720	5,328
Gross Unrealized Depreciation	929	1,358
Fair Value	276,879	273,951
Mortgage-backed securities
HELD-TO-MATURITY
Cost or Amortized Cost	45	48
Gross Unrealized Appreciation	1	1
Gross Unrealized Depreciation	0	0
Fair Value	46	49
AVAILABLE-FOR-SALE
Cost or Amortized Cost	16,366	17,288
Gross Unrealized Appreciation	166	201
Gross Unrealized Depreciation	250	241
Fair Value	16,282	17,248
U.S. Treasury
AVAILABLE-FOR-SALE
Cost or Amortized Cost	22,142	23,216
Gross Unrealized Appreciation	73	87
Gross Unrealized Depreciation	100	108
Fair Value	22,115	23,195
U.S. government agency
AVAILABLE-FOR-SALE
Cost or Amortized Cost	70,671	76,692
Gross Unrealized Appreciation	1,426	1,445
Gross Unrealized Depreciation	438	540
Fair Value	71,659	77,597
States, municipalities and political subdivisions | General obligations: | Midwest
AVAILABLE-FOR-SALE
Cost or Amortized Cost	133,706	143,747
Gross Unrealized Appreciation	1,873	1,808
Gross Unrealized Depreciation	1,068	1,412
Fair Value	134,511	144,143
States, municipalities and political subdivisions | General obligations: | Northeast
AVAILABLE-FOR-SALE
Cost or Amortized Cost	56,241	57,731
Gross Unrealized Appreciation	1,042	909
Gross Unrealized Depreciation	177	231
Fair Value	57,106	58,409
States, municipalities and political subdivisions | General obligations: | South
AVAILABLE-FOR-SALE
Cost or Amortized Cost	144,366	129,475
Gross Unrealized Appreciation	1,455	1,249
Gross Unrealized Depreciation	2,298	2,355
Fair Value	143,523	128,369
States, municipalities and political subdivisions | General obligations: | West
AVAILABLE-FOR-SALE
Cost or Amortized Cost	123,026	114,524
Gross Unrealized Appreciation	1,558	1,380
Gross Unrealized Depreciation	1,839	2,173
Fair Value	122,745	113,731
States, municipalities and political subdivisions | Special revenue: | Midwest
AVAILABLE-FOR-SALE
Cost or Amortized Cost	165,789	167,430
Gross Unrealized Appreciation	2,412	2,313
Gross Unrealized Depreciation	1,326	1,433
Fair Value	166,875	168,310
States, municipalities and political subdivisions | Special revenue: | Northeast
AVAILABLE-FOR-SALE
Cost or Amortized Cost	72,489	70,202
Gross Unrealized Appreciation	558	487
Gross Unrealized Depreciation	2,323	2,624
Fair Value	70,724	68,065
States, municipalities and political subdivisions | Special revenue: | South
AVAILABLE-FOR-SALE
Cost or Amortized Cost	258,835	244,225
Gross Unrealized Appreciation	2,020	1,753
Gross Unrealized Depreciation	5,954	6,791
Fair Value	254,901	239,187
States, municipalities and political subdivisions | Special revenue: | West
AVAILABLE-FOR-SALE
Cost or Amortized Cost	148,390	134,287
Gross Unrealized Appreciation	1,701	1,509
Gross Unrealized Depreciation	3,717	4,052
Fair Value	146,374	131,744
Foreign bonds
AVAILABLE-FOR-SALE
Cost or Amortized Cost	59,295	62,995
Gross Unrealized Appreciation	2,076	2,239
Gross Unrealized Depreciation	0	0
Fair Value	61,371	65,234
Public utilities
AVAILABLE-FOR-SALE
Cost or Amortized Cost	211,412	212,360
Gross Unrealized Appreciation	3,987	3,761
Gross Unrealized Depreciation	427	447
Fair Value	214,972	215,674
Government national mortgage association
AVAILABLE-FOR-SALE
Cost or Amortized Cost	156,554	145,947
Gross Unrealized Appreciation	1,483	1,279
Gross Unrealized Depreciation	2,451	2,766
Fair Value	155,586	144,460
Federal home loan mortgage corporation
AVAILABLE-FOR-SALE
Cost or Amortized Cost	178,052	176,226
Gross Unrealized Appreciation	1,886	1,638
Gross Unrealized Depreciation	3,453	3,406
Fair Value	176,485	174,458
Federal national mortgage association
AVAILABLE-FOR-SALE
Cost or Amortized Cost	99,420	101,414
Gross Unrealized Appreciation	1,907	1,816
Gross Unrealized Depreciation	1,090	1,334
Fair Value	100,237	101,896
Asset-backed securities
AVAILABLE-FOR-SALE
Cost or Amortized Cost	4,367	4,407
Gross Unrealized Appreciation	147	145
Gross Unrealized Depreciation	186	282
Fair Value	4,328	4,270
Equity securities:
AVAILABLE-FOR-SALE
Cost or Amortized Cost	67,977	68,504
Gross Unrealized Appreciation	208,209	202,458
Gross Unrealized Depreciation	502	546
Fair Value	275,684	270,416
Common stocks | Energy
AVAILABLE-FOR-SALE
Cost or Amortized Cost	6,514	6,514
Gross Unrealized Appreciation	7,828	8,555
Gross Unrealized Depreciation	29	22
Fair Value	14,313	15,047
Common stocks | Industrials
AVAILABLE-FOR-SALE
Cost or Amortized Cost	13,117	13,252
Gross Unrealized Appreciation	42,387	38,715
Gross Unrealized Depreciation	175	173
Fair Value	55,329	51,794
Common stocks | Consumer goods and services
AVAILABLE-FOR-SALE
Cost or Amortized Cost	10,074	10,324
Gross Unrealized Appreciation	15,272	13,851
Gross Unrealized Depreciation	64	58
Fair Value	25,282	24,117
Common stocks | Health care
AVAILABLE-FOR-SALE
Cost or Amortized Cost	7,763	7,763
Gross Unrealized Appreciation	22,065	19,657
Gross Unrealized Depreciation	0	0
Fair Value	29,828	27,420
Common stocks | Technology, media and telecommunications
AVAILABLE-FOR-SALE
Cost or Amortized Cost	6,009	5,931
Gross Unrealized Appreciation	9,355	9,476
Gross Unrealized Depreciation	59	38
Fair Value	15,305	15,369
Common stocks | Financial services
AVAILABLE-FOR-SALE
Cost or Amortized Cost	17,069	17,289
Gross Unrealized Appreciation	96,587	98,728
Gross Unrealized Depreciation	50	67
Fair Value	113,606	115,950
Common stocks | Public utilities
AVAILABLE-FOR-SALE
Cost or Amortized Cost	6,394	6,394
Gross Unrealized Appreciation	14,691	13,465
Gross Unrealized Depreciation	125	188
Fair Value	20,960	19,671
Nonredeemable preferred stocks
AVAILABLE-FOR-SALE
Cost or Amortized Cost	1,037	1,037
Gross Unrealized Appreciation	24	11
Gross Unrealized Depreciation	0	0
Fair Value	$ 1,061	$ 1,048