Summary of Investments (Fair Value of Investments) (Details) - USD ($) $ in Thousands	Jun. 30, 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	1,811,473	1,516,610
Gross Unrealized Appreciation	9,555	27,412
Gross Unrealized Depreciation	29,009	8,952
Fair Value	1,792,019	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	1,811,473	2,928,901
Gross Unrealized Appreciation	9,555	52,467
Gross Unrealized Depreciation	29,009	16,273
Fair Value	1,792,019	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	22,477	93,191
Gross Unrealized Appreciation	111	1,972
Gross Unrealized Depreciation	316	110
Fair Value	22,272	95,053
Corporate bonds | Industrials | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	42,286	218,067
Gross Unrealized Appreciation	143	3,881
Gross Unrealized Depreciation	486	241
Fair Value	41,943	221,707
Corporate bonds | Consumer goods and services | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	44,100	183,253
Gross Unrealized Appreciation	106	3,498
Gross Unrealized Depreciation	680	494
Fair Value	43,526	186,257
Corporate bonds | Health care | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	13,071	74,125
Gross Unrealized Appreciation	65	1,312
Gross Unrealized Depreciation	159	29
Fair Value	12,977	75,408
Corporate bonds | Technology, media and telecommunications | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	26,632	146,853
Gross Unrealized Appreciation	43	2,376
Gross Unrealized Depreciation	790	250
Fair Value	25,885	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	73,501	277,824
Gross Unrealized Appreciation	182	5,769
Gross Unrealized Depreciation	1,746	442
Fair Value	71,937	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	8,382	13,828
Gross Unrealized Appreciation	28	101
Gross Unrealized Depreciation	223	238
Fair Value	8,187	13,691
U.S. Treasury | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	120,272	17,073
Gross Unrealized Appreciation	23	4
Gross Unrealized Depreciation	341	186
Fair Value	119,954	16,891
U.S. government agency | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	196,348	121,574
Gross Unrealized Appreciation	633	1,311
Gross Unrealized Depreciation	2,712	717
Fair Value	194,269	122,168
States, municipalities and political subdivisions | General obligations: | Midwest | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	99,821	107,689
Gross Unrealized Appreciation	893	2,446
Gross Unrealized Depreciation	1,053	439
Fair Value	99,661	109,696
States, municipalities and political subdivisions | General obligations: | Northeast | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	40,913	47,477
Gross Unrealized Appreciation	385	1,174
Gross Unrealized Depreciation	187	10
Fair Value	41,111	48,641
States, municipalities and political subdivisions | General obligations: | South | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	119,745	139,870
Gross Unrealized Appreciation	612	2,462
Gross Unrealized Depreciation	2,040	813
Fair Value	118,317	141,519
States, municipalities and political subdivisions | General obligations: | West | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	110,147	111,123
Gross Unrealized Appreciation	916	2,351
Gross Unrealized Depreciation	1,492	463
Fair Value	109,571	113,011
States, municipalities and political subdivisions | Special revenue: | Midwest | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	143,606	155,475
Gross Unrealized Appreciation	1,368	3,620
Gross Unrealized Depreciation	1,208	351
Fair Value	143,766	158,744
States, municipalities and political subdivisions | Special revenue: | Northeast | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	63,889	79,028
Gross Unrealized Appreciation	426	1,351
Gross Unrealized Depreciation	1,431	619
Fair Value	62,884	79,760
States, municipalities and political subdivisions | Special revenue: | South | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	247,056	260,145
Gross Unrealized Appreciation	1,414	5,218
Gross Unrealized Depreciation	4,457	1,851
Fair Value	244,013	263,512
States, municipalities and political subdivisions | Special revenue: | West | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	147,388	156,576
Gross Unrealized Appreciation	1,251	2,929
Gross Unrealized Depreciation	2,469	1,198
Fair Value	146,170	158,307
Foreign bonds | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	7,751	51,361
Gross Unrealized Appreciation	72	1,441
Gross Unrealized Depreciation	0	49
Fair Value	7,823	52,753
Public utilities | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	49,629	206,028
Gross Unrealized Appreciation	196	3,386
Gross Unrealized Depreciation	1,141	270
Fair Value	48,684	209,144
Government national mortgage association | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	73,482	157,836
Gross Unrealized Appreciation	181	1,921
Gross Unrealized Depreciation	2,650	2,274
Fair Value	71,013	157,483
Federal home loan mortgage corporation | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	111,082	201,320
Gross Unrealized Appreciation	136	1,879
Gross Unrealized Depreciation	2,044	4,047
Fair Value	109,174	199,152
Federal national mortgage association | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	46,657	104,903
Gross Unrealized Appreciation	46	1,703
Gross Unrealized Depreciation	1,305	1,174
Fair Value	45,398	105,432
Asset-backed securities | Continuing and Discontinuing Operations
Fixed maturities:
Amortized Cost	3,238	4,282
Gross Unrealized Appreciation	325	362
Gross Unrealized Depreciation	79	8
Fair Value	$ 3,484	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