Summary of Investments (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
HELD-TO-MATURITY
Amortized Cost	$ 0	$ 150
Gross Unrealized Appreciation		0
Gross Unrealized Depreciation		0
Fair Value	0	150
Fixed maturities:
Amortized Cost	1,764,380	1,516,610
Gross Unrealized Appreciation	5,633	27,412
Gross Unrealized Depreciation	39,982	8,952
Fair Value	1,730,031	1,535,070
Equity securities:
Cost or Amortized Cost		57,387
Gross Unrealized Appreciation		224,065
Gross Unrealized Depreciation		539
Fair Value		280,913
Discontinued Operations
HELD-TO-MATURITY
Amortized Cost		34
Gross Unrealized Appreciation		0
Gross Unrealized Depreciation		0
Fair Value		34
Fixed maturities:
Amortized Cost	0	1,412,291
Gross Unrealized Appreciation	0	25,055
Gross Unrealized Depreciation	0	7,321
Fair Value	0	1,430,025
Equity securities:
Cost or Amortized Cost		5,099
Gross Unrealized Appreciation		18,631
Gross Unrealized Depreciation		77
Fair Value		23,653
Continuing and Discontinuing Operations
HELD-TO-MATURITY
Amortized Cost		184
Gross Unrealized Appreciation		0
Gross Unrealized Depreciation		0
Fair Value		184
Fixed maturities:
Amortized Cost	1,764,380	2,928,901
Gross Unrealized Appreciation	5,633	52,467
Gross Unrealized Depreciation	39,982	16,273
Fair Value	1,730,031	2,965,095
Equity securities:
Cost or Amortized Cost		62,486
Gross Unrealized Appreciation		242,696
Gross Unrealized Depreciation		616
Fair Value		304,566
Cost or Amortized Cost		2,991,387
Gross Unrealized Appreciation		295,163
Gross Unrealized Depreciation		16,889
Available-for-sale securities, fair value		3,269,661
Corporate bonds | Energy | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	26,929	93,191
Gross Unrealized Appreciation	77	1,972
Gross Unrealized Depreciation	320	110
Fair Value	26,686	95,053
Corporate bonds | Industrials | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	56,127	218,067
Gross Unrealized Appreciation	103	3,881
Gross Unrealized Depreciation	699	241
Fair Value	55,531	221,707
Corporate bonds | Consumer goods and services | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	52,398	183,253
Gross Unrealized Appreciation	42	3,498
Gross Unrealized Depreciation	763	494
Fair Value	51,677	186,257
Corporate bonds | Health care | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	18,062	74,125
Gross Unrealized Appreciation	58	1,312
Gross Unrealized Depreciation	134	29
Fair Value	17,986	75,408
Corporate bonds | Technology, media and telecommunications | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	26,622	146,853
Gross Unrealized Appreciation	34	2,376
Gross Unrealized Depreciation	534	250
Fair Value	26,122	148,979
Corporate bonds | Financial services | Continuing and Discontinuing Operations
HELD-TO-MATURITY
Amortized Cost		150
Gross Unrealized Appreciation		0
Gross Unrealized Depreciation		0
Fair Value		150
Fixed maturities:
Amortized Cost	77,788	277,824
Gross Unrealized Appreciation	164	5,769
Gross Unrealized Depreciation	1,888	442
Fair Value	76,064	283,151
Mortgage-backed securities | Continuing and Discontinuing Operations
HELD-TO-MATURITY
Amortized Cost		34
Gross Unrealized Appreciation		0
Gross Unrealized Depreciation		0
Fair Value		34
Fixed maturities:
Amortized Cost	7,985	13,828
Gross Unrealized Appreciation	13	101
Gross Unrealized Depreciation	260	238
Fair Value	7,738	13,691
U.S. Treasury | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	27,195	17,073
Gross Unrealized Appreciation	0	4
Gross Unrealized Depreciation	354	186
Fair Value	26,841	16,891
U.S. government agency | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	212,542	121,574
Gross Unrealized Appreciation	310	1,311
Gross Unrealized Depreciation	5,862	717
Fair Value	206,990	122,168
States, municipalities and political subdivisions | General obligations: | Midwest | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	99,023	107,689
Gross Unrealized Appreciation	574	2,446
Gross Unrealized Depreciation	1,359	439
Fair Value	98,238	109,696
States, municipalities and political subdivisions | General obligations: | Northeast | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	37,389	47,477
Gross Unrealized Appreciation	251	1,174
Gross Unrealized Depreciation	289	10
Fair Value	37,351	48,641
States, municipalities and political subdivisions | General obligations: | South | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	115,440	139,870
Gross Unrealized Appreciation	333	2,462
Gross Unrealized Depreciation	2,559	813
Fair Value	113,214	141,519
States, municipalities and political subdivisions | General obligations: | West | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	107,993	111,123
Gross Unrealized Appreciation	643	2,351
Gross Unrealized Depreciation	1,998	463
Fair Value	106,638	113,011
States, municipalities and political subdivisions | Special revenue: | Midwest | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	140,245	155,475
Gross Unrealized Appreciation	817	3,620
Gross Unrealized Depreciation	1,735	351
Fair Value	139,327	158,744
States, municipalities and political subdivisions | Special revenue: | Northeast | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	63,758	79,028
Gross Unrealized Appreciation	240	1,351
Gross Unrealized Depreciation	1,901	619
Fair Value	62,097	79,760
States, municipalities and political subdivisions | Special revenue: | South | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	241,007	260,145
Gross Unrealized Appreciation	611	5,218
Gross Unrealized Depreciation	6,177	1,851
Fair Value	235,441	263,512
States, municipalities and political subdivisions | Special revenue: | West | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	145,072	156,576
Gross Unrealized Appreciation	737	2,929
Gross Unrealized Depreciation	3,411	1,198
Fair Value	142,398	158,307
Foreign bonds | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	9,722	51,361
Gross Unrealized Appreciation	49	1,441
Gross Unrealized Depreciation	24	49
Fair Value	9,747	52,753
Public utilities | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	54,606	206,028
Gross Unrealized Appreciation	129	3,386
Gross Unrealized Depreciation	1,191	270
Fair Value	53,544	209,144
Government national mortgage association | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	78,797	157,836
Gross Unrealized Appreciation	121	1,921
Gross Unrealized Depreciation	3,602	2,274
Fair Value	75,316	157,483
Federal home loan mortgage corporation | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	108,666	201,320
Gross Unrealized Appreciation	7	1,879
Gross Unrealized Depreciation	2,957	4,047
Fair Value	105,716	199,152
Federal national mortgage association | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	53,776	104,903
Gross Unrealized Appreciation	2	1,703
Gross Unrealized Depreciation	1,924	1,174
Fair Value	51,854	105,432
Asset-backed securities | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	3,238	4,282
Gross Unrealized Appreciation	318	362
Gross Unrealized Depreciation	41	8
Fair Value	$ 3,515	4,636
Common stocks | Public utilities | Continuing and Discontinuing Operations
Equity securities:
Cost or Amortized Cost		6,394
Gross Unrealized Appreciation		16,075
Gross Unrealized Depreciation		30
Fair Value		22,439
Common stocks | Energy | Continuing and Discontinuing Operations
Equity securities:
Cost or Amortized Cost		6,514
Gross Unrealized Appreciation		8,171
Gross Unrealized Depreciation		120
Fair Value		14,565
Common stocks | Industrials | Continuing and Discontinuing Operations
Equity securities:
Cost or Amortized Cost		13,117
Gross Unrealized Appreciation		53,522
Gross Unrealized Depreciation		120
Fair Value		66,519
Common stocks | Consumer goods and services | Continuing and Discontinuing Operations
Equity securities:
Cost or Amortized Cost		10,110
Gross Unrealized Appreciation		15,742
Gross Unrealized Depreciation		164
Fair Value		25,688
Common stocks | Health care | Continuing and Discontinuing Operations
Equity securities:
Cost or Amortized Cost		7,763
Gross Unrealized Appreciation		32,340
Gross Unrealized Depreciation		0
Fair Value		40,103
Common stocks | Technology, media and telecommunications | Continuing and Discontinuing Operations
Equity securities:
Cost or Amortized Cost		6,067
Gross Unrealized Appreciation		11,556
Gross Unrealized Depreciation		115
Fair Value		17,508
Common stocks | Financial services | Continuing and Discontinuing Operations
Equity securities:
Cost or Amortized Cost		11,529
Gross Unrealized Appreciation		104,985
Gross Unrealized Depreciation		67
Fair Value		116,447
Nonredeemable preferred stocks | Continuing and Discontinuing Operations
Equity securities:
Cost or Amortized Cost		992
Gross Unrealized Appreciation		305
Gross Unrealized Depreciation		0
Fair Value		$ 1,297