Summary of Investments (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
HELD-TO-MATURITY
Cost or Amortized Cost	$ 652	$ 672
Fair Value	655	675
AVAILABLE-FOR-SALE
Cost or Amortized Cost	2,868,983	2,861,583
Gross Unrealized Appreciation	303,588	227,716
Gross Unrealized Depreciation	5,006	28,091
Fair Value	3,167,565	3,061,208
Fixed Maturities
HELD-TO-MATURITY
Cost or Amortized Cost	652	672
Gross Unrealized Appreciation	3	3
Gross Unrealized Depreciation	0	0
Fair Value	655	675
AVAILABLE-FOR-SALE
Cost or Amortized Cost	2,801,074	2,793,069
Gross Unrealized Appreciation	119,799	58,994
Gross Unrealized Depreciation	4,409	27,102
Fair Value	2,916,464	2,824,961
Corporate bonds | Energy
AVAILABLE-FOR-SALE
Cost or Amortized Cost	110,345	116,800
Gross Unrealized Appreciation	3,747	1,032
Gross Unrealized Depreciation	187	4,713
Fair Value	113,905	113,119
Corporate bonds | Industrials
AVAILABLE-FOR-SALE
Cost or Amortized Cost	232,088	227,589
Gross Unrealized Appreciation	12,192	3,329
Gross Unrealized Depreciation	668	6,663
Fair Value	243,612	224,255
Corporate bonds | Consumer goods and services
AVAILABLE-FOR-SALE
Cost or Amortized Cost	179,502	172,529
Gross Unrealized Appreciation	9,422	2,844
Gross Unrealized Depreciation	0	776
Fair Value	188,924	174,597
Corporate bonds | Health care
AVAILABLE-FOR-SALE
Cost or Amortized Cost	82,497	92,132
Gross Unrealized Appreciation	4,928	2,168
Gross Unrealized Depreciation	0	791
Fair Value	87,425	93,509
Corporate bonds | Technology, media and telecommunications
HELD-TO-MATURITY
Cost or Amortized Cost	450	450
Gross Unrealized Appreciation	2	1
Gross Unrealized Depreciation	0	0
Fair Value	452	451
AVAILABLE-FOR-SALE
Cost or Amortized Cost	141,598	142,431
Gross Unrealized Appreciation	6,225	1,972
Gross Unrealized Depreciation	168	2,003
Fair Value	147,655	142,400
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	266,576	259,382
Gross Unrealized Appreciation	12,262	5,246
Gross Unrealized Depreciation	20	1,143
Fair Value	278,818	263,485
Mortgage-backed securities
HELD-TO-MATURITY
Cost or Amortized Cost	52	72
Gross Unrealized Appreciation	1	2
Gross Unrealized Depreciation	0	0
Fair Value	53	74
AVAILABLE-FOR-SALE
Cost or Amortized Cost	18,422	16,413
Gross Unrealized Appreciation	465	376
Gross Unrealized Depreciation	8	51
Fair Value	18,879	16,738
U.S. Treasury
AVAILABLE-FOR-SALE
Cost or Amortized Cost	21,966	21,587
Gross Unrealized Appreciation	281	100
Gross Unrealized Depreciation	1	38
Fair Value	22,246	21,649
U.S. government agency
AVAILABLE-FOR-SALE
Cost or Amortized Cost	71,717	232,808
Gross Unrealized Appreciation	2,893	2,622
Gross Unrealized Depreciation	3	2,400
Fair Value	74,607	233,030
States, municipalities and political subdivisions | General obligations: | Midwest
AVAILABLE-FOR-SALE
Cost or Amortized Cost	153,150	160,484
Gross Unrealized Appreciation	5,302	4,990
Gross Unrealized Depreciation	90	18
Fair Value	158,362	165,456
States, municipalities and political subdivisions | General obligations: | Northeast
AVAILABLE-FOR-SALE
Cost or Amortized Cost	58,373	56,449
Gross Unrealized Appreciation	2,836	1,996
Gross Unrealized Depreciation	0	0
Fair Value	61,209	58,445
States, municipalities and political subdivisions | General obligations: | South
AVAILABLE-FOR-SALE
Cost or Amortized Cost	132,843	125,565
Gross Unrealized Appreciation	4,444	3,358
Gross Unrealized Depreciation	314	134
Fair Value	136,973	128,789
States, municipalities and political subdivisions | General obligations: | West
AVAILABLE-FOR-SALE
Cost or Amortized Cost	123,845	103,721
Gross Unrealized Appreciation	4,631	3,160
Gross Unrealized Depreciation	180	67
Fair Value	128,296	106,814
States, municipalities and political subdivisions | Special revenue: | Midwest
AVAILABLE-FOR-SALE
Cost or Amortized Cost	161,368	152,780
Gross Unrealized Appreciation	6,799	4,956
Gross Unrealized Depreciation	56	30
Fair Value	168,111	157,706
States, municipalities and political subdivisions | Special revenue: | Northeast
AVAILABLE-FOR-SALE
Cost or Amortized Cost	54,109	23,892
Gross Unrealized Appreciation	1,863	919
Gross Unrealized Depreciation	98	212
Fair Value	55,874	24,599
States, municipalities and political subdivisions | Special revenue: | South
AVAILABLE-FOR-SALE
Cost or Amortized Cost	208,747	144,183
Gross Unrealized Appreciation	7,399	4,281
Gross Unrealized Depreciation	542	27
Fair Value	215,604	148,437
States, municipalities and political subdivisions | Special revenue: | West
AVAILABLE-FOR-SALE
Cost or Amortized Cost	100,225	78,935
Gross Unrealized Appreciation	4,069	3,150
Gross Unrealized Depreciation	184	44
Fair Value	104,110	82,041
Foreign bonds
AVAILABLE-FOR-SALE
Cost or Amortized Cost	71,011	82,580
Gross Unrealized Appreciation	3,484	2,405
Gross Unrealized Depreciation	894	2,457
Fair Value	73,601	82,528
Public utilities
AVAILABLE-FOR-SALE
Cost or Amortized Cost	217,670	213,233
Gross Unrealized Appreciation	10,539	3,701
Gross Unrealized Depreciation	83	1,251
Fair Value	228,126	215,683
Government national mortgage association
AVAILABLE-FOR-SALE
Cost or Amortized Cost	131,200	120,220
Gross Unrealized Appreciation	4,635	1,391
Gross Unrealized Depreciation	289	1,985
Fair Value	135,546	119,626
Federal home loan mortgage corporation
AVAILABLE-FOR-SALE
Cost or Amortized Cost	153,379	137,874
Gross Unrealized Appreciation	5,626	2,377
Gross Unrealized Depreciation	262	1,342
Fair Value	158,743	138,909
Federal national mortgage association
AVAILABLE-FOR-SALE
Cost or Amortized Cost	105,952	106,021
Gross Unrealized Appreciation	5,439	2,400
Gross Unrealized Depreciation	131	941
Fair Value	111,260	107,480
Asset-backed securities
AVAILABLE-FOR-SALE
Cost or Amortized Cost	4,491	5,461
Gross Unrealized Appreciation	318	221
Gross Unrealized Depreciation	231	16
Fair Value	4,578	5,666
Equity securities:
AVAILABLE-FOR-SALE
Cost or Amortized Cost	67,909	68,514
Gross Unrealized Appreciation	183,789	168,722
Gross Unrealized Depreciation	597	989
Fair Value	251,101	236,247
Common stocks | Energy
AVAILABLE-FOR-SALE
Cost or Amortized Cost	6,514	6,103
Gross Unrealized Appreciation	7,988	5,374
Gross Unrealized Depreciation	39	266
Fair Value	14,463	11,211
Common stocks | Industrials
AVAILABLE-FOR-SALE
Cost or Amortized Cost	13,252	13,251
Gross Unrealized Appreciation	34,387	31,872
Gross Unrealized Depreciation	215	313
Fair Value	47,424	44,810
Common stocks | Consumer goods and services
AVAILABLE-FOR-SALE
Cost or Amortized Cost	10,324	10,301
Gross Unrealized Appreciation	15,059	13,017
Gross Unrealized Depreciation	43	3
Fair Value	25,340	23,315
Common stocks | Health care
AVAILABLE-FOR-SALE
Cost or Amortized Cost	7,763	7,763
Gross Unrealized Appreciation	21,100	20,454
Gross Unrealized Depreciation	0	0
Fair Value	28,863	28,217
Common stocks | Technology, media and telecommunications
AVAILABLE-FOR-SALE
Cost or Amortized Cost	5,931	5,931
Gross Unrealized Appreciation	9,214	7,538
Gross Unrealized Depreciation	37	105
Fair Value	15,108	13,364
Common stocks | Financial services
AVAILABLE-FOR-SALE
Cost or Amortized Cost	17,288	17,392
Gross Unrealized Appreciation	82,294	78,411
Gross Unrealized Depreciation	65	109
Fair Value	99,517	95,694
Common stocks | Public utilities
AVAILABLE-FOR-SALE
Cost or Amortized Cost	6,394	7,231
Gross Unrealized Appreciation	13,718	12,022
Gross Unrealized Depreciation	198	193
Fair Value	19,914	19,060
Nonredeemable preferred stocks
AVAILABLE-FOR-SALE
Cost or Amortized Cost	443	542
Gross Unrealized Appreciation	29	34
Gross Unrealized Depreciation	0	0
Fair Value	$ 472	$ 576