Summary of Investments (Investments in Unrealized Loss Position) (Details) $ in Thousands	Sep. 30, 2017 USD ($) issue	Dec. 31, 2016 USD ($) issue
Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	141	567
12 months or longer | issue	154	55
Fair Value
Less than 12 months	$ 350,445	$ 1,060,938
12 months or longer	380,378	53,798
Total	730,823	1,114,736
Gross Unrealized Depreciation
Less than 12 months	4,144	32,167
12 months or longer	12,741	2,598
Total	$ 16,885	$ 34,765
U.S. Treasury | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	3	9
12 months or longer | issue	3	0
Fair Value
Less than 12 months	$ 8,920	$ 10,800
12 months or longer	2,829	0
Total	11,749	10,800
Gross Unrealized Depreciation
Less than 12 months	44	108
12 months or longer	47	0
Total	$ 91	$ 108
U.S. government agency | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	8	10
12 months or longer | issue	3	0
Fair Value
Less than 12 months	$ 33,607	$ 36,593
12 months or longer	12,789	0
Total	46,396	36,593
Gross Unrealized Depreciation
Less than 12 months	306	540
12 months or longer	210	0
Total	$ 516	$ 540
Fixed Maturities
Number of Issues
Less than 12 months | issue	85	404
12 months or longer | issue	101	12
Fair Value
Less than 12 months	$ 206,874	$ 654,235
12 months or longer	259,297	6,288
Total	466,171	660,523
Gross Unrealized Depreciation
Less than 12 months	2,059	23,359
12 months or longer	8,615	315
Total	$ 10,674	$ 23,674
Fixed Maturities | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	135	554
12 months or longer | issue	140	33
Fair Value
Less than 12 months	$ 349,318	$ 1,060,577
12 months or longer	379,458	53,078
Total	728,776	1,113,655
Gross Unrealized Depreciation
Less than 12 months	3,896	32,104
12 months or longer	12,298	2,115
Total	$ 16,194	$ 34,219
Fixed Maturities | Discontinued Operations
Number of Issues
Less than 12 months | issue	50	150
12 months or longer | issue	39	21
Fair Value
Less than 12 months	$ 142,444	$ 406,342
12 months or longer	120,161	46,790
Total	262,605	453,132
Gross Unrealized Depreciation
Less than 12 months	1,837	8,745
12 months or longer	3,683	1,800
Total	$ 5,520	$ 10,545
States, municipalities and political subdivisions | General obligations: | Midwest | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	3	27
12 months or longer | issue	3	0
Fair Value
Less than 12 months	$ 4,343	$ 40,545
12 months or longer	19,742	0
Total	24,085	40,545
Gross Unrealized Depreciation
Less than 12 months	46	1,412
12 months or longer	453	0
Total	$ 499	$ 1,412
States, municipalities and political subdivisions | General obligations: | Northeast | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	0	9
12 months or longer | issue	1	0
Fair Value
Less than 12 months	$ 0	$ 9,874
12 months or longer	3,587	0
Total	3,587	9,874
Gross Unrealized Depreciation
Less than 12 months	0	231
12 months or longer	73	0
Total	$ 73	$ 231
States, municipalities and political subdivisions | General obligations: | South | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	7	37
12 months or longer | issue	11	0
Fair Value
Less than 12 months	$ 14,594	$ 53,699
12 months or longer	27,919	0
Total	42,513	53,699
Gross Unrealized Depreciation
Less than 12 months	56	2,355
12 months or longer	1,154	0
Total	$ 1,210	$ 2,355
States, municipalities and political subdivisions | General obligations: | West | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	2	30
12 months or longer | issue	8	0
Fair Value
Less than 12 months	$ 3,600	$ 55,265
12 months or longer	25,333	0
Total	28,933	55,265
Gross Unrealized Depreciation
Less than 12 months	29	2,173
12 months or longer	934	0
Total	$ 963	$ 2,173
States, municipalities and political subdivisions | Special revenue: | Midwest | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	2	41
12 months or longer | issue	7	0
Fair Value
Less than 12 months	$ 3,990	$ 62,937
12 months or longer	19,034	0
Total	23,024	62,937
Gross Unrealized Depreciation
Less than 12 months	10	1,433
12 months or longer	484	0
Total	$ 494	$ 1,433
States, municipalities and political subdivisions | Special revenue: | Northeast | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	6	22
12 months or longer | issue	9	0
Fair Value
Less than 12 months	$ 27,751	$ 54,993
12 months or longer	15,275	0
Total	43,026	54,993
Gross Unrealized Depreciation
Less than 12 months	312	2,624
12 months or longer	483	0
Total	$ 795	$ 2,624
States, municipalities and political subdivisions | Special revenue: | South | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	13	79
12 months or longer | issue	27	0
Fair Value
Less than 12 months	$ 32,415	$ 152,979
12 months or longer	66,978	0
Total	99,393	152,979
Gross Unrealized Depreciation
Less than 12 months	433	6,791
12 months or longer	2,541	0
Total	$ 2,974	$ 6,791
States, municipalities and political subdivisions | Special revenue: | West | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	8	44
12 months or longer | issue	22	0
Fair Value
Less than 12 months	$ 20,369	$ 81,676
12 months or longer	55,836	0
Total	76,205	81,676
Gross Unrealized Depreciation
Less than 12 months	135	4,052
12 months or longer	1,805	0
Total	$ 1,940	$ 4,052
Public utilities | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	2	20
12 months or longer | issue	4	2
Fair Value
Less than 12 months	$ 3,201	$ 38,511
12 months or longer	7,491	2,122
Total	10,692	40,633
Gross Unrealized Depreciation
Less than 12 months	52	423
12 months or longer	148	24
Total	$ 200	$ 447
Corporate bonds | Energy | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	2	8
12 months or longer | issue	1	3
Fair Value
Less than 12 months	$ 6,166	$ 15,938
12 months or longer	1,807	8,232
Total	7,973	24,170
Gross Unrealized Depreciation
Less than 12 months	13	313
12 months or longer	82	106
Total	$ 95	$ 419
Corporate bonds | Industrials | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	0	24
12 months or longer | issue	2	3
Fair Value
Less than 12 months	$ 0	$ 42,854
12 months or longer	4,271	5,641
Total	4,271	48,495
Gross Unrealized Depreciation
Less than 12 months	0	596
12 months or longer	109	386
Total	$ 109	$ 982
Corporate bonds | Consumer goods and services | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	5	11
12 months or longer | issue	2	0
Fair Value
Less than 12 months	$ 7,469	$ 21,059
12 months or longer	5,097	0
Total	12,566	21,059
Gross Unrealized Depreciation
Less than 12 months	62	295
12 months or longer	73	0
Total	$ 135	$ 295
Corporate bonds | Health care | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue		9
12 months or longer | issue		0
Fair Value
Less than 12 months		$ 20,918
12 months or longer		0
Total		20,918
Gross Unrealized Depreciation
Less than 12 months		151
12 months or longer		0
Total		$ 151
Corporate bonds | Technology, media and telecommunications | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	6	16
12 months or longer | issue	2	3
Fair Value
Less than 12 months	$ 15,263	$ 41,230
12 months or longer	8,384	10,241
Total	23,647	51,471
Gross Unrealized Depreciation
Less than 12 months	88	516
12 months or longer	105	303
Total	$ 193	$ 819
Corporate bonds | Financial services | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	11	37
12 months or longer | issue	1	0
Fair Value
Less than 12 months	$ 21,623	$ 75,286
12 months or longer	7,243	0
Total	28,866	75,286
Gross Unrealized Depreciation
Less than 12 months	163	1,358
12 months or longer	140	0
Total	$ 303	$ 1,358
Mortgage-backed securities | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	11	16
12 months or longer | issue	3	5
Fair Value
Less than 12 months	$ 4,411	$ 9,611
12 months or longer	4,790	1,198
Total	9,201	10,809
Gross Unrealized Depreciation
Less than 12 months	43	187
12 months or longer	136	54
Total	$ 179	$ 241
Government national mortgage association | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	16	36
12 months or longer | issue	13	9
Fair Value
Less than 12 months	$ 40,243	$ 82,430
12 months or longer	38,573	13,603
Total	78,816	96,033
Gross Unrealized Depreciation
Less than 12 months	439	2,261
12 months or longer	1,019	505
Total	$ 1,458	$ 2,766
Federal home loan mortgage corporation | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	16	41
12 months or longer | issue	12	3
Fair Value
Less than 12 months	$ 64,647	$ 105,775
12 months or longer	42,472	5,141
Total	107,119	110,916
Gross Unrealized Depreciation
Less than 12 months	1,166	3,165
12 months or longer	1,966	241
Total	$ 3,132	$ 3,406
Federal national mortgage association | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	13	27
12 months or longer | issue	6	4
Fair Value
Less than 12 months	$ 35,709	$ 46,633
12 months or longer	10,008	4,341
Total	45,717	50,974
Gross Unrealized Depreciation
Less than 12 months	496	1,091
12 months or longer	336	243
Total	$ 832	$ 1,334
Asset-backed securities | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	1	1
12 months or longer | issue	0	1
Fair Value
Less than 12 months	$ 997	$ 971
12 months or longer	0	2,559
Total	997	3,530
Gross Unrealized Depreciation
Less than 12 months	3	29
12 months or longer	0	253
Total	$ 3	$ 282
Common stocks | Public utilities | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	0	0
12 months or longer | issue	1	3
Fair Value
Less than 12 months	$ 0	$ 0
12 months or longer	250	120
Total	250	120
Gross Unrealized Depreciation
Less than 12 months	0	0
12 months or longer	58	188
Total	$ 58	$ 188
Common stocks | Energy | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	2	0
12 months or longer | issue	1	1
Fair Value
Less than 12 months	$ 546	$ 0
12 months or longer	182	163
Total	728	163
Gross Unrealized Depreciation
Less than 12 months	102	0
12 months or longer	4	22
Total	$ 106	$ 22
Common stocks | Industrials | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	1	0
12 months or longer | issue	5	6
Fair Value
Less than 12 months	$ 106	$ 0
12 months or longer	141	239
Total	247	239
Gross Unrealized Depreciation
Less than 12 months	6	0
12 months or longer	158	173
Total	$ 164	$ 173
Common stocks | Consumer goods and services | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	0	3
12 months or longer | issue	4	2
Fair Value
Less than 12 months	$ 0	$ 282
12 months or longer	178	15
Total	178	297
Gross Unrealized Depreciation
Less than 12 months	0	55
12 months or longer	154	3
Total	$ 154	$ 58
Common stocks | Technology, media and telecommunications | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	2	7
12 months or longer | issue	1	8
Fair Value
Less than 12 months	$ 445	$ 26
12 months or longer	4	33
Total	449	59
Gross Unrealized Depreciation
Less than 12 months	115	5
12 months or longer	21	33
Total	$ 136	$ 38
Common stocks | Financial services | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	1	3
12 months or longer | issue	2	2
Fair Value
Less than 12 months	$ 30	$ 53
12 months or longer	165	150
Total	195	203
Gross Unrealized Depreciation
Less than 12 months	25	3
12 months or longer	48	64
Total	$ 73	$ 67
Equity securities:
Number of Issues
Less than 12 months | issue	6	12
12 months or longer | issue	10	17
Fair Value
Less than 12 months	$ 1,127	$ 351
12 months or longer	540	477
Total	1,667	828
Gross Unrealized Depreciation
Less than 12 months	248	62
12 months or longer	343	254
Total	$ 591	$ 316
Equity securities: | Continuing and Discontinuing Operations
Number of Issues
Less than 12 months | issue	6	13
12 months or longer | issue	14	22
Fair Value
Less than 12 months	$ 1,127	$ 361
12 months or longer	920	720
Total	2,047	1,081
Gross Unrealized Depreciation
Less than 12 months	248	63
12 months or longer	443	483
Total	$ 691	$ 546
Equity securities: | Discontinued Operations
Number of Issues
Less than 12 months | issue	0	1
12 months or longer | issue	4	5
Fair Value
Less than 12 months	$ 0	$ 10
12 months or longer	380	243
Total	380	253
Gross Unrealized Depreciation
Less than 12 months	0	1
12 months or longer	100	229
Total	$ 100	$ 230