Summary of Investments (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
HELD-TO-MATURITY
Amortized Cost	$ 150	$ 150
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	0	0
Fair Value	150	150
Fixed maturities:
Amortized Cost	1,577,448	1,516,610
Gross Unrealized Appreciation	11,219	27,412
Gross Unrealized Depreciation	24,671	8,952
Fair Value	1,563,996	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	0	34
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	0	0
Fair Value	0	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	150	184
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	0	0
Fair Value	150	184
Fixed maturities:
Amortized Cost	1,577,448	2,928,901
Gross Unrealized Appreciation	11,219	52,467
Gross Unrealized Depreciation	24,671	16,273
Fair Value	1,563,996	2,965,095
Equity securities:
Cost or Amortized Cost		62,486
Gross Unrealized Appreciation		242,696
Gross Unrealized Depreciation		616
Fair Value		304,566
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	22,516	93,191
Gross Unrealized Appreciation	157	1,972
Gross Unrealized Depreciation	228	110
Fair Value	22,445	95,053
Corporate bonds | Industrials | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	33,214	218,067
Gross Unrealized Appreciation	200	3,881
Gross Unrealized Depreciation	427	241
Fair Value	32,987	221,707
Corporate bonds | Consumer goods and services | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	31,044	183,253
Gross Unrealized Appreciation	217	3,498
Gross Unrealized Depreciation	291	494
Fair Value	30,970	186,257
Corporate bonds | Health care | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	12,170	74,125
Gross Unrealized Appreciation	93	1,312
Gross Unrealized Depreciation	37	29
Fair Value	12,226	75,408
Corporate bonds | Technology, media and telecommunications | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	15,447	146,853
Gross Unrealized Appreciation	74	2,376
Gross Unrealized Depreciation	265	250
Fair Value	15,256	148,979
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
Fixed maturities:
Amortized Cost	55,489	277,824
Gross Unrealized Appreciation	226	5,769
Gross Unrealized Depreciation	1,203	442
Fair Value	54,512	283,151
Mortgage-backed securities | Continuing and Discontinuing Operations
HELD-TO-MATURITY
Amortized Cost	0	34
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	0	0
Fair Value	0	34
Fixed maturities:
Amortized Cost	8,763	13,828
Gross Unrealized Appreciation	71	101
Gross Unrealized Depreciation	152	238
Fair Value	8,682	13,691
U.S. Treasury | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	15,995	17,073
Gross Unrealized Appreciation	1	4
Gross Unrealized Depreciation	270	186
Fair Value	15,726	16,891
U.S. government agency | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	135,379	121,574
Gross Unrealized Appreciation	606	1,311
Gross Unrealized Depreciation	1,556	717
Fair Value	134,429	122,168
States, municipalities and political subdivisions | General obligations: | Midwest | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	105,376	107,689
Gross Unrealized Appreciation	1,019	2,446
Gross Unrealized Depreciation	1,060	439
Fair Value	105,335	109,696
States, municipalities and political subdivisions | General obligations: | Northeast | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	46,132	47,477
Gross Unrealized Appreciation	468	1,174
Gross Unrealized Depreciation	196	10
Fair Value	46,404	48,641
States, municipalities and political subdivisions | General obligations: | South | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	134,796	139,870
Gross Unrealized Appreciation	916	2,462
Gross Unrealized Depreciation	1,993	813
Fair Value	133,719	141,519
States, municipalities and political subdivisions | General obligations: | West | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	112,871	111,123
Gross Unrealized Appreciation	976	2,351
Gross Unrealized Depreciation	1,516	463
Fair Value	112,331	113,011
States, municipalities and political subdivisions | Special revenue: | Midwest | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	143,823	155,475
Gross Unrealized Appreciation	1,495	3,620
Gross Unrealized Depreciation	1,188	351
Fair Value	144,130	158,744
States, municipalities and political subdivisions | Special revenue: | Northeast | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	64,020	79,028
Gross Unrealized Appreciation	481	1,351
Gross Unrealized Depreciation	1,443	619
Fair Value	63,058	79,760
States, municipalities and political subdivisions | Special revenue: | South | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	250,757	260,145
Gross Unrealized Appreciation	1,707	5,218
Gross Unrealized Depreciation	4,404	1,851
Fair Value	248,060	263,512
States, municipalities and political subdivisions | Special revenue: | West | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	149,928	156,576
Gross Unrealized Appreciation	1,294	2,929
Gross Unrealized Depreciation	2,757	1,198
Fair Value	148,465	158,307
Foreign bonds | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	10,826	51,361
Gross Unrealized Appreciation	156	1,441
Gross Unrealized Depreciation	0	49
Fair Value	10,982	52,753
Public utilities | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	48,024	206,028
Gross Unrealized Appreciation	249	3,386
Gross Unrealized Depreciation	632	270
Fair Value	47,641	209,144
Government national mortgage association | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	74,000	157,836
Gross Unrealized Appreciation	280	1,921
Gross Unrealized Depreciation	2,086	2,274
Fair Value	72,194	157,483
Federal home loan mortgage corporation | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	56,578	201,320
Gross Unrealized Appreciation	73	1,879
Gross Unrealized Depreciation	1,822	4,047
Fair Value	54,829	199,152
Federal national mortgage association | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	47,104	104,903
Gross Unrealized Appreciation	142	1,703
Gross Unrealized Depreciation	1,100	1,174
Fair Value	46,146	105,432
Asset-backed securities | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	3,196	4,282
Gross Unrealized Appreciation	318	362
Gross Unrealized Depreciation	45	8
Fair Value	$ 3,469	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