Summary of Investments (Investments in Unrealized Loss Position) (Details) $ in Thousands	Jun. 30, 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	7
12 months or longer | issue	2
Less than 12 months | issue		5
12 months or longer | issue		2
Fair Value
Less than 12 months	$ 65,396
12 months or longer	4,687
Total	70,083
Less than 12 months		$ 10,370
12 months or longer		5,765
Total		16,135
Gross Unrealized Depreciation
Less than 12 months	201
12 months or longer	140
Total	$ 341
Gross Unrealized Depreciation
Less than 12 months		67
12 months or longer		119
Total		$ 186
U.S. government agency | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	26
12 months or longer | issue	2
Less than 12 months | issue		11
12 months or longer | issue		5
Fair Value
Less than 12 months	$ 126,009
12 months or longer	7,644
Total	133,653
Less than 12 months		$ 64,842
12 months or longer		19,372
Total		84,214
Gross Unrealized Depreciation
Less than 12 months	2,357
12 months or longer	355
Total	$ 2,712
Gross Unrealized Depreciation
Less than 12 months		390
12 months or longer		327
Total		$ 717
Fixed Maturities
Number of Issues
Less than 12 months | issue	324	88
12 months or longer | issue	121	112
Fair Value
Less than 12 months	$ 744,296	$ 232,489
12 months or longer	296,165	302,815
Total	1,040,461	535,304
Gross Unrealized Depreciation
Less than 12 months	14,292	1,791
12 months or longer	14,717	7,161
Total	$ 29,009	$ 8,952
Fixed Maturities | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	324
12 months or longer | issue	121
Less than 12 months | issue	324	162
12 months or longer | issue	121	173
Fair Value
Less than 12 months	$ 744,296
12 months or longer	296,165
Total	1,040,461
Less than 12 months	744,296	$ 413,380
12 months or longer	296,165	470,440
Total	1,040,461	883,820
Gross Unrealized Depreciation
Less than 12 months	14,292
12 months or longer	14,717
Total	29,009
Gross Unrealized Depreciation
Less than 12 months	14,292	3,206
12 months or longer	14,717	13,067
Total	$ 29,009	$ 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	13
12 months or longer | issue	3
Less than 12 months | issue		2
12 months or longer | issue		3
Fair Value
Less than 12 months	$ 19,600
12 months or longer	19,177
Total	38,777
Less than 12 months		$ 2,177
12 months or longer		19,729
Total		21,906
Gross Unrealized Depreciation
Less than 12 months	140
12 months or longer	913
Total	$ 1,053
Gross Unrealized Depreciation
Less than 12 months		8
12 months or longer		431
Total		$ 439
States, municipalities and political subdivisions | General obligations: | Northeast | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	4
12 months or longer | issue	1
Less than 12 months | issue		0
12 months or longer | issue		1
Fair Value
Less than 12 months	$ 13,208
12 months or longer	3,534
Total	16,742
Less than 12 months		$ 0
12 months or longer		3,644
Total		3,644
Gross Unrealized Depreciation
Less than 12 months	78
12 months or longer	109
Total	$ 187
Gross Unrealized Depreciation
Less than 12 months		0
12 months or longer		10
Total		$ 10
States, municipalities and political subdivisions | General obligations: | South | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	17
12 months or longer | issue	11
Less than 12 months | issue		3
12 months or longer | issue		11
Fair Value
Less than 12 months	$ 35,461
12 months or longer	28,623
Total	64,084
Less than 12 months		$ 7,959
12 months or longer		29,545
Total		37,504
Gross Unrealized Depreciation
Less than 12 months	503
12 months or longer	1,537
Total	$ 2,040
Gross Unrealized Depreciation
Less than 12 months		32
12 months or longer		781
Total		$ 813
States, municipalities and political subdivisions | General obligations: | West | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	12
12 months or longer | issue	8
Less than 12 months | issue		2
12 months or longer | issue		8
Fair Value
Less than 12 months	$ 28,125
12 months or longer	24,959
Total	53,084
Less than 12 months		$ 5,944
12 months or longer		25,755
Total		31,699
Gross Unrealized Depreciation
Less than 12 months	388
12 months or longer	1,104
Total	$ 1,492
Gross Unrealized Depreciation
Less than 12 months		18
12 months or longer		445
Total		$ 463
States, municipalities and political subdivisions | Special revenue: | Midwest | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	18
12 months or longer | issue	7
Less than 12 months | issue		2
12 months or longer | issue		7
Fair Value
Less than 12 months	$ 33,830
12 months or longer	18,510
Total	52,340
Less than 12 months		$ 3,486
12 months or longer		19,130
Total		22,616
Gross Unrealized Depreciation
Less than 12 months	356
12 months or longer	852
Total	$ 1,208
Gross Unrealized Depreciation
Less than 12 months		15
12 months or longer		336
Total		$ 351
States, municipalities and political subdivisions | Special revenue: | Northeast | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	4
12 months or longer | issue	11
Less than 12 months | issue		1
12 months or longer | issue		11
Fair Value
Less than 12 months	$ 12,472
12 months or longer	27,636
Total	40,108
Less than 12 months		$ 4,471
12 months or longer		28,476
Total		32,947
Gross Unrealized Depreciation
Less than 12 months	120
12 months or longer	1,311
Total	$ 1,431
Gross Unrealized Depreciation
Less than 12 months		37
12 months or longer		582
Total		$ 619
States, municipalities and political subdivisions | Special revenue: | South | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	30
12 months or longer | issue	26
Less than 12 months | issue		8
12 months or longer | issue		27
Fair Value
Less than 12 months	$ 61,106
12 months or longer	65,618
Total	126,724
Less than 12 months		$ 7,749
12 months or longer		69,917
Total		77,666
Gross Unrealized Depreciation
Less than 12 months	1,034
12 months or longer	3,423
Total	$ 4,457
Gross Unrealized Depreciation
Less than 12 months		107
12 months or longer		1,744
Total		$ 1,851
States, municipalities and political subdivisions | Special revenue: | West | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	17
12 months or longer | issue	21
Less than 12 months | issue		3
12 months or longer | issue		22
Fair Value
Less than 12 months	$ 31,156
12 months or longer	52,290
Total	83,446
Less than 12 months		$ 5,424
12 months or longer		56,753
Total		62,177
Gross Unrealized Depreciation
Less than 12 months	237
12 months or longer	2,232
Total	$ 2,469
Gross Unrealized Depreciation
Less than 12 months		16
12 months or longer		1,182
Total		$ 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	19
12 months or longer | issue	0
Less than 12 months | issue		8
12 months or longer | issue		5
Fair Value
Less than 12 months	$ 39,933
12 months or longer	0
Total	39,933
Less than 12 months		$ 19,186
12 months or longer		8,446
Total		27,632
Gross Unrealized Depreciation
Less than 12 months	1,141
12 months or longer	0
Total	$ 1,141
Gross Unrealized Depreciation
Less than 12 months		79
12 months or longer		191
Total		$ 270
Corporate bonds | Energy | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	7
12 months or longer | issue	0
Less than 12 months | issue		1
12 months or longer | issue		1
Fair Value
Less than 12 months	$ 10,465
12 months or longer	0
Total	10,465
Less than 12 months		$ 2,236
12 months or longer		1,606
Total		3,842
Gross Unrealized Depreciation
Less than 12 months	316
12 months or longer	0
Total	$ 316
Gross Unrealized Depreciation
Less than 12 months		13
12 months or longer		97
Total		$ 110
Corporate bonds | Industrials | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	8
12 months or longer | issue	0
Less than 12 months | issue		10
12 months or longer | issue		2
Fair Value
Less than 12 months	$ 22,206
12 months or longer	0
Total	22,206
Less than 12 months		$ 27,773
12 months or longer		4,275
Total		32,048
Gross Unrealized Depreciation
Less than 12 months	486
12 months or longer	0
Total	$ 486
Gross Unrealized Depreciation
Less than 12 months		146
12 months or longer		95
Total		$ 241
Corporate bonds | Consumer goods and services | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	14
12 months or longer | issue	0
Less than 12 months | issue		14
12 months or longer | issue		3
Fair Value
Less than 12 months	$ 28,226
12 months or longer	0
Total	28,226
Less than 12 months		$ 32,781
12 months or longer		6,813
Total		39,594
Gross Unrealized Depreciation
Less than 12 months	680
12 months or longer	0
Total	$ 680
Gross Unrealized Depreciation
Less than 12 months		248
12 months or longer		246
Total		$ 494
Corporate bonds | Health care | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	4
12 months or longer | issue	0
Less than 12 months | issue		4
12 months or longer | issue		0
Fair Value
Less than 12 months	$ 6,489
12 months or longer	0
Total	6,489
Less than 12 months		$ 9,947
12 months or longer		0
Total		9,947
Gross Unrealized Depreciation
Less than 12 months	159
12 months or longer	0
Total	$ 159
Gross Unrealized Depreciation
Less than 12 months		29
12 months or longer		0
Total		$ 29
Corporate bonds | Technology, media and telecommunications | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	10
12 months or longer | issue	0
Less than 12 months | issue		12
12 months or longer | issue		3
Fair Value
Less than 12 months	$ 22,147
12 months or longer	0
Total	22,147
Less than 12 months		$ 35,319
12 months or longer		10,413
Total		45,732
Gross Unrealized Depreciation
Less than 12 months	790
12 months or longer	0
Total	$ 790
Gross Unrealized Depreciation
Less than 12 months		122
12 months or longer		128
Total		$ 250
Corporate bonds | Financial services | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	21
12 months or longer | issue	1
Less than 12 months | issue		22
12 months or longer | issue		4
Fair Value
Less than 12 months	$ 49,917
12 months or longer	5,293
Total	55,210
Less than 12 months		$ 50,144
12 months or longer		11,389
Total		61,533
Gross Unrealized Depreciation
Less than 12 months	1,440
12 months or longer	306
Total	$ 1,746
Gross Unrealized Depreciation
Less than 12 months		256
12 months or longer		186
Total		$ 442
Mortgage-backed securities | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	34
12 months or longer | issue	11
Less than 12 months | issue		10
12 months or longer | issue		10
Fair Value
Less than 12 months	$ 4,217
12 months or longer	2,012
Total	6,229
Less than 12 months		$ 2,458
12 months or longer		6,641
Total		9,099
Gross Unrealized Depreciation
Less than 12 months	113
12 months or longer	110
Total	$ 223
Gross Unrealized Depreciation
Less than 12 months		18
12 months or longer		220
Total		$ 238
Government national mortgage association | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	21
12 months or longer | issue	7
Less than 12 months | issue		20
12 months or longer | issue		17
Fair Value
Less than 12 months	$ 50,094
12 months or longer	11,744
Total	61,838
Less than 12 months		$ 49,764
12 months or longer		46,969
Total		96,733
Gross Unrealized Depreciation
Less than 12 months	1,766
12 months or longer	884
Total	$ 2,650
Gross Unrealized Depreciation
Less than 12 months		629
12 months or longer		1,645
Total		$ 2,274
Federal home loan mortgage corporation | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	20
12 months or longer | issue	7
Less than 12 months | issue		11
12 months or longer | issue		20
Fair Value
Less than 12 months	$ 46,374
12 months or longer	17,556
Total	63,930
Less than 12 months		$ 37,543
12 months or longer		75,679
Total		113,222
Gross Unrealized Depreciation
Less than 12 months	964
12 months or longer	1,080
Total	$ 2,044
Gross Unrealized Depreciation
Less than 12 months		577
12 months or longer		3,470
Total		$ 4,047
Federal national mortgage association | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	17
12 months or longer | issue	3
Less than 12 months | issue		11
12 months or longer | issue		11
Fair Value
Less than 12 months	$ 35,019
12 months or longer	6,882
Total	41,901
Less than 12 months		$ 31,958
12 months or longer		20,123
Total		52,081
Gross Unrealized Depreciation
Less than 12 months	944
12 months or longer	361
Total	$ 1,305
Gross Unrealized Depreciation
Less than 12 months		342
12 months or longer		832
Total		$ 1,174
Asset-backed securities | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	1
12 months or longer | issue	0
Less than 12 months | issue		1
12 months or longer | issue		0
Fair Value
Less than 12 months	$ 2,846
12 months or longer	0
Total	2,846
Less than 12 months		$ 992
12 months or longer		0
Total		992
Gross Unrealized Depreciation
Less than 12 months	79
12 months or longer	0
Total	$ 79
Gross Unrealized Depreciation
Less than 12 months		8
12 months or longer		0
Total		$ 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