Summary of Investments (Investments in Unrealized Loss Position) (Details) $ in Thousands	Mar. 31, 2018 USD ($) issue	Dec. 31, 2017 USD ($) issue
Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue		169
12 months or longer | issue		183
Fair Value
Less than 12 months		$ 414,666
12 months or longer		471,075
Total		885,741
Gross Unrealized Depreciation
Less than 12 months		3,489
12 months or longer		13,400
Total		$ 16,889
U.S. Treasury | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	5	5
12 months or longer | issue	1	2
Fair Value
Less than 12 months	$ 10,263	$ 10,370
12 months or longer	4,711	5,765
Total	14,974	16,135
Gross Unrealized Depreciation
Less than 12 months	141	67
12 months or longer	129	119
Total	$ 270	$ 186
U.S. government agency | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	15	11
12 months or longer | issue	2	5
Fair Value
Less than 12 months	$ 81,651	$ 64,842
12 months or longer	7,675	19,372
Total	89,326	84,214
Gross Unrealized Depreciation
Less than 12 months	1,232	390
12 months or longer	324	327
Total	$ 1,556	$ 717
Fixed Maturities
Number of Issues
Less than 12 months | issue	241	88
12 months or longer | issue	114	112
Fair Value
Less than 12 months	$ 572,149	$ 232,489
12 months or longer	294,481	302,815
Total	866,630	535,304
Gross Unrealized Depreciation
Less than 12 months	10,271	1,791
12 months or longer	14,400	7,161
Total	$ 24,671	$ 8,952
Fixed Maturities | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	241	162
12 months or longer | issue	114	173
Fair Value
Less than 12 months	$ 572,149	$ 413,380
12 months or longer	294,481	470,440
Total	866,630	883,820
Gross Unrealized Depreciation
Less than 12 months	10,271	3,206
12 months or longer	14,400	13,067
Total	$ 24,671	$ 16,273
Fixed Maturities | Discontinued Operations
Number of Issues
Less than 12 months | issue	0	74
12 months or longer | issue	0	61
Fair Value
Less than 12 months	$ 0	$ 180,891
12 months or longer	0	167,625
Total	0	348,516
Gross Unrealized Depreciation
Less than 12 months	0	1,415
12 months or longer	0	5,906
Total	$ 0	$ 7,321
States, municipalities and political subdivisions | General obligations: | Midwest | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	12	2
12 months or longer | issue	3	3
Fair Value
Less than 12 months	$ 15,917	$ 2,177
12 months or longer	19,209	19,729
Total	35,126	21,906
Gross Unrealized Depreciation
Less than 12 months	144	8
12 months or longer	916	431
Total	$ 1,060	$ 439
States, municipalities and political subdivisions | General obligations: | Northeast | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	4	0
12 months or longer | issue	1	1
Fair Value
Less than 12 months	$ 13,214	$ 0
12 months or longer	3,546	3,644
Total	16,760	3,644
Gross Unrealized Depreciation
Less than 12 months	94	0
12 months or longer	102	10
Total	$ 196	$ 10
States, municipalities and political subdivisions | General obligations: | South | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	18	3
12 months or longer | issue	11	11
Fair Value
Less than 12 months	$ 38,686	$ 7,959
12 months or longer	28,716	29,545
Total	67,402	37,504
Gross Unrealized Depreciation
Less than 12 months	466	32
12 months or longer	1,527	781
Total	$ 1,993	$ 813
States, municipalities and political subdivisions | General obligations: | West | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	13	2
12 months or longer | issue	8	8
Fair Value
Less than 12 months	$ 31,285	$ 5,944
12 months or longer	25,021	25,755
Total	56,306	31,699
Gross Unrealized Depreciation
Less than 12 months	406	18
12 months or longer	1,110	445
Total	$ 1,516	$ 463
States, municipalities and political subdivisions | Special revenue: | Midwest | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	20	2
12 months or longer | issue	7	7
Fair Value
Less than 12 months	$ 37,637	$ 3,486
12 months or longer	18,561	19,130
Total	56,198	22,616
Gross Unrealized Depreciation
Less than 12 months	334	15
12 months or longer	854	336
Total	$ 1,188	$ 351
States, municipalities and political subdivisions | Special revenue: | Northeast | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	4	1
12 months or longer | issue	11	11
Fair Value
Less than 12 months	$ 12,466	$ 4,471
12 months or longer	27,722	28,476
Total	40,188	32,947
Gross Unrealized Depreciation
Less than 12 months	162	37
12 months or longer	1,281	582
Total	$ 1,443	$ 619
States, municipalities and political subdivisions | Special revenue: | South | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	24	8
12 months or longer | issue	27	27
Fair Value
Less than 12 months	$ 50,787	$ 7,749
12 months or longer	67,940	69,917
Total	118,727	77,666
Gross Unrealized Depreciation
Less than 12 months	866	107
12 months or longer	3,538	1,744
Total	$ 4,404	$ 1,851
States, municipalities and political subdivisions | Special revenue: | West | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	16	3
12 months or longer | issue	21	22
Fair Value
Less than 12 months	$ 30,075	$ 5,424
12 months or longer	52,243	56,753
Total	82,318	62,177
Gross Unrealized Depreciation
Less than 12 months	331	16
12 months or longer	2,426	1,182
Total	$ 2,757	$ 1,198
Foreign bonds
Number of Issues
Less than 12 months | issue		1
12 months or longer | issue		0
Fair Value
Less than 12 months		$ 857
12 months or longer		0
Total		857
Gross Unrealized Depreciation
Less than 12 months		49
12 months or longer		0
Total		$ 49
Public utilities | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	13	8
12 months or longer | issue	0	5
Fair Value
Less than 12 months	$ 33,171	$ 19,186
12 months or longer	0	8,446
Total	33,171	27,632
Gross Unrealized Depreciation
Less than 12 months	632	79
12 months or longer	0	191
Total	$ 632	$ 270
Corporate bonds | Energy | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	5	1
12 months or longer | issue	0	1
Fair Value
Less than 12 months	$ 10,567	$ 2,236
12 months or longer	0	1,606
Total	10,567	3,842
Gross Unrealized Depreciation
Less than 12 months	228	13
12 months or longer	0	97
Total	$ 228	$ 110
Corporate bonds | Industrials | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	6	10
12 months or longer | issue	0	2
Fair Value
Less than 12 months	$ 20,349	$ 27,773
12 months or longer	0	4,275
Total	20,349	32,048
Gross Unrealized Depreciation
Less than 12 months	427	146
12 months or longer	0	95
Total	$ 427	$ 241
Corporate bonds | Consumer goods and services | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	5	14
12 months or longer | issue	0	3
Fair Value
Less than 12 months	$ 15,823	$ 32,781
12 months or longer	0	6,813
Total	15,823	39,594
Gross Unrealized Depreciation
Less than 12 months	291	248
12 months or longer	0	246
Total	$ 291	$ 494
Corporate bonds | Health care | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	2	4
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 3,694	$ 9,947
12 months or longer	0	0
Total	3,694	9,947
Gross Unrealized Depreciation
Less than 12 months	37	29
12 months or longer	0	0
Total	$ 37	$ 29
Corporate bonds | Technology, media and telecommunications | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	3	12
12 months or longer | issue	0	3
Fair Value
Less than 12 months	$ 9,745	$ 35,319
12 months or longer	0	10,413
Total	9,745	45,732
Gross Unrealized Depreciation
Less than 12 months	265	122
12 months or longer	0	128
Total	$ 265	$ 250
Corporate bonds | Financial services | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	13	22
12 months or longer | issue	1	4
Fair Value
Less than 12 months	$ 34,617	$ 50,144
12 months or longer	5,374	11,389
Total	39,991	61,533
Gross Unrealized Depreciation
Less than 12 months	977	256
12 months or longer	226	186
Total	$ 1,203	$ 442
Mortgage-backed securities | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	16	10
12 months or longer | issue	10	10
Fair Value
Less than 12 months	$ 2,828	$ 2,458
12 months or longer	2,123	6,641
Total	4,951	9,099
Gross Unrealized Depreciation
Less than 12 months	70	18
12 months or longer	82	220
Total	$ 152	$ 238
Government national mortgage association | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	20	20
12 months or longer | issue	6	17
Fair Value
Less than 12 months	$ 47,898	$ 49,764
12 months or longer	12,087	46,969
Total	59,985	96,733
Gross Unrealized Depreciation
Less than 12 months	1,351	629
12 months or longer	735	1,645
Total	$ 2,086	$ 2,274
Federal home loan mortgage corporation | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	13	11
12 months or longer | issue	4	20
Fair Value
Less than 12 months	$ 35,124	$ 37,543
12 months or longer	12,590	75,679
Total	47,714	113,222
Gross Unrealized Depreciation
Less than 12 months	1,050	577
12 months or longer	772	3,470
Total	$ 1,822	$ 4,047
Federal national mortgage association | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	13	11
12 months or longer | issue	1	11
Fair Value
Less than 12 months	$ 33,515	$ 31,958
12 months or longer	6,963	20,123
Total	40,478	52,081
Gross Unrealized Depreciation
Less than 12 months	722	342
12 months or longer	378	832
Total	$ 1,100	$ 1,174
Asset-backed securities | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	1	1
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 2,837	$ 992
12 months or longer	0	0
Total	2,837	992
Gross Unrealized Depreciation
Less than 12 months	45	8
12 months or longer	0	0
Total	$ 45	$ 8
Common stocks | Public utilities | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue		0
12 months or longer | issue		1
Fair Value
Less than 12 months		$ 0
12 months or longer		278
Total		278
Gross Unrealized Depreciation
Less than 12 months		0
12 months or longer		30
Total		$ 30
Common stocks | Energy | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue		2
12 months or longer | issue		0
Fair Value
Less than 12 months		$ 528
12 months or longer		0
Total		528
Gross Unrealized Depreciation
Less than 12 months		120
12 months or longer		0
Total		$ 120
Common stocks | Industrials | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue		1
12 months or longer | issue		5
Fair Value
Less than 12 months		$ 99
12 months or longer		193
Total		292
Gross Unrealized Depreciation
Less than 12 months		13
12 months or longer		107
Total		$ 120
Common stocks | Consumer goods and services | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue		0
12 months or longer | issue		2
Fair Value
Less than 12 months		$ 0
12 months or longer		151
Total		151
Gross Unrealized Depreciation
Less than 12 months		0
12 months or longer		164
Total		$ 164
Common stocks | Technology, media and telecommunications | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue		2
12 months or longer | issue		1
Fair Value
Less than 12 months		$ 466
12 months or longer		4
Total		470
Gross Unrealized Depreciation
Less than 12 months		95
12 months or longer		20
Total		$ 115
Common stocks | Financial services | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue		2
12 months or longer | issue		1
Fair Value
Less than 12 months		$ 193
12 months or longer		9
Total		202
Gross Unrealized Depreciation
Less than 12 months		55
12 months or longer		12
Total		$ 67
Equity securities:
Number of Issues
Less than 12 months | issue		5
12 months or longer | issue		6
Fair Value
Less than 12 months		$ 1,129
12 months or longer		385
Total		1,514
Gross Unrealized Depreciation
Less than 12 months		236
12 months or longer		303
Total		$ 539
Equity securities: | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue		7
12 months or longer | issue		10
Fair Value
Less than 12 months		$ 1,286
12 months or longer		635
Total		1,921
Gross Unrealized Depreciation
Less than 12 months		283
12 months or longer		333
Total		$ 616
Equity securities: | Discontinued Operations
Number of Issues
Less than 12 months | issue		2
12 months or longer | issue		4
Fair Value
Less than 12 months		$ 157
12 months or longer		250
Total		407
Gross Unrealized Depreciation
Less than 12 months		47
12 months or longer		30
Total		$ 77