Summary of Investments (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
HELD-TO-MATURITY
Amortized Cost	$ 150	$ 150
Fair Value	150	150
Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	2,942,681	2,956,009
Gross Unrealized Appreciation	291,814	247,298
Gross Unrealized Depreciation	16,885	34,765
Fair Value	3,217,610	3,168,542
Fixed Maturities
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	1,480,730	1,458,235
Gross Unrealized Appreciation	28,606	18,725
Gross Unrealized Depreciation	10,674	23,674
Fair Value	1,498,662	1,453,286
Fixed Maturities | Discontinued Operations
HELD-TO-MATURITY
Amortized Cost	38	48
Gross Unrealized Appreciation	1	1
Gross Unrealized Depreciation	0	0
Fair Value	39	49
AVAILABLE-FOR-SALE
Amortized Cost	1,399,465	1,429,270
Gross Unrealized Appreciation	33,046	26,115
Gross Unrealized Depreciation	5,520	10,545
Fair Value	1,426,991	1,444,840
Fixed Maturities | Continuing and Discontinuing Operations
HELD-TO-MATURITY
Amortized Cost	188	198
Gross Unrealized Appreciation	1	1
Gross Unrealized Depreciation	0	0
Fair Value	189	199
AVAILABLE-FOR-SALE
Amortized Cost	2,880,195	2,887,505
Gross Unrealized Appreciation	61,652	44,840
Gross Unrealized Depreciation	16,194	34,219
Fair Value	2,925,653	2,898,126
Corporate bonds | Energy | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	96,373	107,084
Gross Unrealized Appreciation	2,367	2,195
Gross Unrealized Depreciation	95	419
Fair Value	98,645	108,860
Corporate bonds | Industrials | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	209,076	225,526
Gross Unrealized Appreciation	5,323	5,359
Gross Unrealized Depreciation	109	982
Fair Value	214,290	229,903
Corporate bonds | Consumer goods and services | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	181,471	178,135
Gross Unrealized Appreciation	5,049	3,847
Gross Unrealized Depreciation	135	295
Fair Value	186,385	181,687
Corporate bonds | Health care | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	75,775	81,211
Gross Unrealized Appreciation	2,600	2,063
Gross Unrealized Depreciation	0	151
Fair Value	78,375	83,123
Corporate bonds | Technology, media and telecommunications | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	143,024	143,402
Gross Unrealized Appreciation	3,308	2,029
Gross Unrealized Depreciation	193	819
Fair Value	146,139	144,612
Corporate bonds | Financial services | Continuing and Discontinuing Operations
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	252,373	269,981
Gross Unrealized Appreciation	6,939	5,328
Gross Unrealized Depreciation	303	1,358
Fair Value	259,009	273,951
Mortgage-backed securities | Continuing and Discontinuing Operations
HELD-TO-MATURITY
Amortized Cost	38	48
Gross Unrealized Appreciation	1	1
Gross Unrealized Depreciation	0	0
Fair Value	39	49
AVAILABLE-FOR-SALE
Amortized Cost	14,496	17,288
Gross Unrealized Appreciation	169	201
Gross Unrealized Depreciation	179	241
Fair Value	14,486	17,248
U.S. Treasury | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	22,032	23,216
Gross Unrealized Appreciation	40	87
Gross Unrealized Depreciation	91	108
Fair Value	21,981	23,195
U.S. government agency | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	98,523	76,692
Gross Unrealized Appreciation	1,518	1,445
Gross Unrealized Depreciation	516	540
Fair Value	99,525	77,597
States, municipalities and political subdivisions | General obligations: | Midwest | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	120,549	143,747
Gross Unrealized Appreciation	2,388	1,808
Gross Unrealized Depreciation	499	1,412
Fair Value	122,438	144,143
States, municipalities and political subdivisions | General obligations: | Northeast | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	50,174	57,731
Gross Unrealized Appreciation	1,478	909
Gross Unrealized Depreciation	73	231
Fair Value	51,579	58,409
States, municipalities and political subdivisions | General obligations: | South | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	142,172	129,475
Gross Unrealized Appreciation	2,463	1,249
Gross Unrealized Depreciation	1,210	2,355
Fair Value	143,425	128,369
States, municipalities and political subdivisions | General obligations: | West | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	113,135	114,524
Gross Unrealized Appreciation	2,474	1,380
Gross Unrealized Depreciation	963	2,173
Fair Value	114,646	113,731
States, municipalities and political subdivisions | Special revenue: | Midwest | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	151,634	167,430
Gross Unrealized Appreciation	3,646	2,313
Gross Unrealized Depreciation	494	1,433
Fair Value	154,786	168,310
States, municipalities and political subdivisions | Special revenue: | Northeast | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	79,159	70,202
Gross Unrealized Appreciation	1,061	487
Gross Unrealized Depreciation	795	2,624
Fair Value	79,425	68,065
States, municipalities and political subdivisions | Special revenue: | South | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	261,141	244,225
Gross Unrealized Appreciation	4,421	1,753
Gross Unrealized Depreciation	2,974	6,791
Fair Value	262,588	239,187
States, municipalities and political subdivisions | Special revenue: | West | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	157,622	134,287
Gross Unrealized Appreciation	2,676	1,509
Gross Unrealized Depreciation	1,940	4,052
Fair Value	158,358	131,744
Foreign bonds | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	54,300	62,995
Gross Unrealized Appreciation	1,907	2,239
Gross Unrealized Depreciation	0	0
Fair Value	56,207	65,234
Public utilities | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	201,418	212,360
Gross Unrealized Appreciation	4,538	3,761
Gross Unrealized Depreciation	200	447
Fair Value	205,756	215,674
Government national mortgage association | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	153,896	145,947
Gross Unrealized Appreciation	2,292	1,279
Gross Unrealized Depreciation	1,458	2,766
Fair Value	154,730	144,460
Federal home loan mortgage corporation | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	191,246	176,226
Gross Unrealized Appreciation	2,410	1,638
Gross Unrealized Depreciation	3,132	3,406
Fair Value	190,524	174,458
Federal national mortgage association | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	106,326	101,414
Gross Unrealized Appreciation	2,240	1,816
Gross Unrealized Depreciation	832	1,334
Fair Value	107,734	101,896
Asset-backed securities | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	4,280	4,407
Gross Unrealized Appreciation	345	145
Gross Unrealized Depreciation	3	282
Fair Value	4,622	4,270
Equity securities:
AVAILABLE-FOR-SALE
Amortized Cost	57,387	59,994
Gross Unrealized Appreciation	212,545	186,692
Gross Unrealized Depreciation	591	316
Fair Value	269,341	246,370
Equity securities: | Discontinued Operations
AVAILABLE-FOR-SALE
Amortized Cost	5,099	8,510
Gross Unrealized Appreciation	17,617	15,766
Gross Unrealized Depreciation	100	230
Fair Value	22,616	24,046
Equity securities: | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	62,486	68,504
Gross Unrealized Appreciation	230,162	202,458
Gross Unrealized Depreciation	691	546
Fair Value	291,957	270,416
Common stocks | Public utilities | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	6,394	6,394
Gross Unrealized Appreciation	15,750	13,465
Gross Unrealized Depreciation	58	188
Fair Value	22,086	19,671
Common stocks | Energy | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	6,514	6,514
Gross Unrealized Appreciation	7,998	8,555
Gross Unrealized Depreciation	106	22
Fair Value	14,406	15,047
Common stocks | Industrials | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	13,117	13,252
Gross Unrealized Appreciation	49,890	38,715
Gross Unrealized Depreciation	164	173
Fair Value	62,843	51,794
Common stocks | Consumer goods and services | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	10,070	10,324
Gross Unrealized Appreciation	14,872	13,851
Gross Unrealized Depreciation	154	58
Fair Value	24,788	24,117
Common stocks | Health care | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	7,763	7,763
Gross Unrealized Appreciation	29,463	19,657
Gross Unrealized Depreciation	0	0
Fair Value	37,226	27,420
Common stocks | Technology, media and telecommunications | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	6,006	5,931
Gross Unrealized Appreciation	10,215	9,476
Gross Unrealized Depreciation	136	38
Fair Value	16,085	15,369
Common stocks | Financial services | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	11,630	17,289
Gross Unrealized Appreciation	101,813	98,728
Gross Unrealized Depreciation	73	67
Fair Value	113,370	115,950
Nonredeemable preferred stocks | Continuing and Discontinuing Operations
AVAILABLE-FOR-SALE
Amortized Cost	992	1,037
Gross Unrealized Appreciation	161	11
Gross Unrealized Depreciation	0	0
Fair Value	$ 1,153	$ 1,048