Summary of Investments (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
HELD-TO-MATURITY
Amortized Cost	$ 192	$ 198
Fair Value	192	199
AVAILABLE-FOR-SALE
Amortized Cost	2,974,859	2,956,009
Gross Unrealized Appreciation	275,588	247,298
Gross Unrealized Depreciation	19,062	34,765
Fair Value	3,231,385	3,168,542
Fixed Maturities
HELD-TO-MATURITY
Amortized Cost	192	198
Gross Unrealized Appreciation	0	1
Gross Unrealized Depreciation	0	0
Fair Value	192	199
AVAILABLE-FOR-SALE
Amortized Cost	2,912,326	2,887,505
Gross Unrealized Appreciation	61,097	44,840
Gross Unrealized Depreciation	18,344	34,219
Fair Value	2,955,079	2,898,126
Corporate bonds | Energy
AVAILABLE-FOR-SALE
Amortized Cost	99,416	107,084
Gross Unrealized Appreciation	2,249	2,195
Gross Unrealized Depreciation	200	419
Fair Value	101,465	108,860
Corporate bonds | Industrials
AVAILABLE-FOR-SALE
Amortized Cost	214,919	225,526
Gross Unrealized Appreciation	5,726	5,359
Gross Unrealized Depreciation	219	982
Fair Value	220,426	229,903
Corporate bonds | Consumer goods and services
AVAILABLE-FOR-SALE
Amortized Cost	179,992	178,135
Gross Unrealized Appreciation	4,927	3,847
Gross Unrealized Depreciation	157	295
Fair Value	184,762	181,687
Corporate bonds | Health care
AVAILABLE-FOR-SALE
Amortized Cost	75,863	81,211
Gross Unrealized Appreciation	2,469	2,063
Gross Unrealized Depreciation	0	151
Fair Value	78,332	83,123
Corporate bonds | Technology, media and telecommunications
AVAILABLE-FOR-SALE
Amortized Cost	143,209	143,402
Gross Unrealized Appreciation	3,224	2,029
Gross Unrealized Depreciation	175	819
Fair Value	146,258	144,612
Corporate bonds | Financial services
HELD-TO-MATURITY
Amortized Cost	150	150
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	0	0
Fair Value	150	150
AVAILABLE-FOR-SALE
Amortized Cost	261,314	269,981
Gross Unrealized Appreciation	6,763	5,328
Gross Unrealized Depreciation	526	1,358
Fair Value	267,551	273,951
Mortgage-backed securities
HELD-TO-MATURITY
Amortized Cost	42	48
Gross Unrealized Appreciation	0	1
Gross Unrealized Depreciation	0	0
Fair Value	42	49
AVAILABLE-FOR-SALE
Amortized Cost	15,515	17,288
Gross Unrealized Appreciation	178	201
Gross Unrealized Depreciation	195	241
Fair Value	15,498	17,248
U.S. Treasury
AVAILABLE-FOR-SALE
Amortized Cost	22,068	23,216
Gross Unrealized Appreciation	56	87
Gross Unrealized Depreciation	80	108
Fair Value	22,044	23,195
U.S. government agency
AVAILABLE-FOR-SALE
Amortized Cost	79,643	76,692
Gross Unrealized Appreciation	1,574	1,445
Gross Unrealized Depreciation	424	540
Fair Value	80,793	77,597
States, municipalities and political subdivisions | General obligations: | Midwest
AVAILABLE-FOR-SALE
Amortized Cost	128,921	143,747
Gross Unrealized Appreciation	2,588	1,808
Gross Unrealized Depreciation	555	1,412
Fair Value	130,954	144,143
States, municipalities and political subdivisions | General obligations: | Northeast
AVAILABLE-FOR-SALE
Amortized Cost	54,322	57,731
Gross Unrealized Appreciation	1,348	909
Gross Unrealized Depreciation	91	231
Fair Value	55,579	58,409
States, municipalities and political subdivisions | General obligations: | South
AVAILABLE-FOR-SALE
Amortized Cost	141,845	129,475
Gross Unrealized Appreciation	2,496	1,249
Gross Unrealized Depreciation	1,350	2,355
Fair Value	142,991	128,369
States, municipalities and political subdivisions | General obligations: | West
AVAILABLE-FOR-SALE
Amortized Cost	120,651	114,524
Gross Unrealized Appreciation	2,468	1,380
Gross Unrealized Depreciation	1,165	2,173
Fair Value	121,954	113,731
States, municipalities and political subdivisions | Special revenue: | Midwest
AVAILABLE-FOR-SALE
Amortized Cost	163,411	167,430
Gross Unrealized Appreciation	3,579	2,313
Gross Unrealized Depreciation	678	1,433
Fair Value	166,312	168,310
States, municipalities and political subdivisions | Special revenue: | Northeast
AVAILABLE-FOR-SALE
Amortized Cost	80,282	70,202
Gross Unrealized Appreciation	926	487
Gross Unrealized Depreciation	1,229	2,624
Fair Value	79,979	68,065
States, municipalities and political subdivisions | Special revenue: | South
AVAILABLE-FOR-SALE
Amortized Cost	258,666	244,225
Gross Unrealized Appreciation	3,871	1,753
Gross Unrealized Depreciation	3,547	6,791
Fair Value	258,990	239,187
States, municipalities and political subdivisions | Special revenue: | West
AVAILABLE-FOR-SALE
Amortized Cost	152,224	134,287
Gross Unrealized Appreciation	2,614	1,509
Gross Unrealized Depreciation	2,377	4,052
Fair Value	152,461	131,744
Foreign bonds
AVAILABLE-FOR-SALE
Amortized Cost	55,503	62,995
Gross Unrealized Appreciation	1,987	2,239
Gross Unrealized Depreciation	0	0
Fair Value	57,490	65,234
Public utilities
AVAILABLE-FOR-SALE
Amortized Cost	207,437	212,360
Gross Unrealized Appreciation	4,474	3,761
Gross Unrealized Depreciation	240	447
Fair Value	211,671	215,674
Government national mortgage association
AVAILABLE-FOR-SALE
Amortized Cost	155,129	145,947
Gross Unrealized Appreciation	2,228	1,279
Gross Unrealized Depreciation	1,490	2,766
Fair Value	155,867	144,460
Federal home loan mortgage corporation
AVAILABLE-FOR-SALE
Amortized Cost	194,556	176,226
Gross Unrealized Appreciation	2,566	1,638
Gross Unrealized Depreciation	2,894	3,406
Fair Value	194,228	174,458
Federal national mortgage association
AVAILABLE-FOR-SALE
Amortized Cost	103,061	101,414
Gross Unrealized Appreciation	2,367	1,816
Gross Unrealized Depreciation	752	1,334
Fair Value	104,676	101,896
Asset-backed securities
AVAILABLE-FOR-SALE
Amortized Cost	4,379	4,407
Gross Unrealized Appreciation	419	145
Gross Unrealized Depreciation	0	282
Fair Value	4,798	4,270
Equity securities:
AVAILABLE-FOR-SALE
Amortized Cost	62,533	68,504
Gross Unrealized Appreciation	214,491	202,458
Gross Unrealized Depreciation	718	546
Fair Value	276,306	270,416
Common stocks | Energy
AVAILABLE-FOR-SALE
Amortized Cost	6,514	6,514
Gross Unrealized Appreciation	7,133	8,555
Gross Unrealized Depreciation	121	22
Fair Value	13,526	15,047
Common stocks | Industrials
AVAILABLE-FOR-SALE
Amortized Cost	13,116	13,252
Gross Unrealized Appreciation	44,356	38,715
Gross Unrealized Depreciation	181	173
Fair Value	57,291	51,794
Common stocks | Consumer goods and services
AVAILABLE-FOR-SALE
Amortized Cost	10,070	10,324
Gross Unrealized Appreciation	15,379	13,851
Gross Unrealized Depreciation	118	58
Fair Value	25,331	24,117
Common stocks | Health care
AVAILABLE-FOR-SALE
Amortized Cost	7,763	7,763
Gross Unrealized Appreciation	24,958	19,657
Gross Unrealized Depreciation	0	0
Fair Value	32,721	27,420
Common stocks | Technology, media and telecommunications
AVAILABLE-FOR-SALE
Amortized Cost	6,009	5,931
Gross Unrealized Appreciation	8,837	9,476
Gross Unrealized Depreciation	107	38
Fair Value	14,739	15,369
Common stocks | Financial services
AVAILABLE-FOR-SALE
Amortized Cost	11,630	17,289
Gross Unrealized Appreciation	98,733	98,728
Gross Unrealized Depreciation	51	67
Fair Value	110,312	115,950
Common stocks | Public utilities
AVAILABLE-FOR-SALE
Amortized Cost	6,394	6,394
Gross Unrealized Appreciation	15,062	13,465
Gross Unrealized Depreciation	140	188
Fair Value	21,316	19,671
Nonredeemable preferred stocks
AVAILABLE-FOR-SALE
Amortized Cost	1,037	1,037
Gross Unrealized Appreciation	33	11
Gross Unrealized Depreciation	0	0
Fair Value	$ 1,070	$ 1,048