Summary of Investments (Investments in Unrealized Loss Position) (Details) $ in Thousands	Mar. 31, 2017 USD ($) issue	Dec. 31, 2016 USD ($) issue
Number of Issues
Less than 12 months | issue	466	567
12 months or longer | issue	55	55
Fair Value
Less than 12 months	$ 925,223	$ 1,060,938
12 months or longer	52,451	53,798
Total	977,674	1,114,736
Gross Unrealized Depreciation
Less than 12 months	27,857	32,167
12 months or longer	2,368	2,598
Total	$ 30,225	$ 34,765
U.S. Treasury
Number of Issues
Less than 12 months | issue	11	9
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 11,779	$ 10,800
12 months or longer	0	0
Total	11,779	10,800
Gross Unrealized Depreciation
Less than 12 months	100	108
12 months or longer	0	0
Total	$ 100	$ 108
U.S. government agency
Number of Issues
Less than 12 months | issue	9	10
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 33,651	$ 36,593
12 months or longer	0	0
Total	33,651	36,593
Gross Unrealized Depreciation
Less than 12 months	438	540
12 months or longer	0	0
Total	$ 438	$ 540
Fixed Maturities
Number of Issues
Less than 12 months | issue	453	554
12 months or longer | issue	31	33
Fair Value
Less than 12 months	$ 924,865	$ 1,060,577
12 months or longer	51,417	53,078
Total	976,282	1,113,655
Gross Unrealized Depreciation
Less than 12 months	27,836	32,104
12 months or longer	1,887	2,115
Total	$ 29,723	$ 34,219
States, municipalities and political subdivisions | General obligations: | Midwest
Number of Issues
Less than 12 months | issue	16	27
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 32,139	$ 40,545
12 months or longer	0	0
Total	32,139	40,545
Gross Unrealized Depreciation
Less than 12 months	1,068	1,412
12 months or longer	0	0
Total	$ 1,068	$ 1,412
States, municipalities and political subdivisions | General obligations: | Northeast
Number of Issues
Less than 12 months | issue	4	9
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 6,939	$ 9,874
12 months or longer	0	0
Total	6,939	9,874
Gross Unrealized Depreciation
Less than 12 months	177	231
12 months or longer	0	0
Total	$ 177	$ 231
States, municipalities and political subdivisions | General obligations: | South
Number of Issues
Less than 12 months | issue	28	37
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 60,252	$ 53,699
12 months or longer	0	0
Total	60,252	53,699
Gross Unrealized Depreciation
Less than 12 months	2,298	2,355
12 months or longer	0	0
Total	$ 2,298	$ 2,355
States, municipalities and political subdivisions | General obligations: | West
Number of Issues
Less than 12 months | issue	23	30
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 48,967	$ 55,265
12 months or longer	0	0
Total	48,967	55,265
Gross Unrealized Depreciation
Less than 12 months	1,839	2,173
12 months or longer	0	0
Total	$ 1,839	$ 2,173
States, municipalities and political subdivisions | Special revenue: | Midwest
Number of Issues
Less than 12 months | issue	30	41
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 51,512	$ 62,937
12 months or longer	0	0
Total	51,512	62,937
Gross Unrealized Depreciation
Less than 12 months	1,326	1,433
12 months or longer	0	0
Total	$ 1,326	$ 1,433
States, municipalities and political subdivisions | Special revenue: | Northeast
Number of Issues
Less than 12 months | issue	21	22
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 51,512	$ 54,993
12 months or longer	0	0
Total	51,512	54,993
Gross Unrealized Depreciation
Less than 12 months	2,323	2,624
12 months or longer	0	0
Total	$ 2,323	$ 2,624
States, municipalities and political subdivisions | Special revenue: | South
Number of Issues
Less than 12 months | issue	69	79
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 142,601	$ 152,979
12 months or longer	0	0
Total	142,601	152,979
Gross Unrealized Depreciation
Less than 12 months	5,954	6,791
12 months or longer	0	0
Total	$ 5,954	$ 6,791
States, municipalities and political subdivisions | Special revenue: | West
Number of Issues
Less than 12 months | issue	36	44
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 75,008	$ 81,676
12 months or longer	0	0
Total	75,008	81,676
Gross Unrealized Depreciation
Less than 12 months	3,717	4,052
12 months or longer	0	0
Total	$ 3,717	$ 4,052
Public utilities
Number of Issues
Less than 12 months | issue	17	20
12 months or longer | issue	3	2
Fair Value
Less than 12 months	$ 32,195	$ 38,511
12 months or longer	2,107	2,122
Total	34,302	40,633
Gross Unrealized Depreciation
Less than 12 months	389	423
12 months or longer	38	24
Total	$ 427	$ 447
Corporate bonds | Energy
Number of Issues
Less than 12 months | issue	4	8
12 months or longer | issue	1	3
Fair Value
Less than 12 months	$ 6,951	$ 15,938
12 months or longer	4,174	8,232
Total	11,125	24,170
Gross Unrealized Depreciation
Less than 12 months	173	313
12 months or longer	20	106
Total	$ 193	$ 419
Corporate bonds | Industrials
Number of Issues
Less than 12 months | issue	16	24
12 months or longer | issue	2	3
Fair Value
Less than 12 months	$ 26,794	$ 42,854
12 months or longer	4,758	5,641
Total	31,552	48,495
Gross Unrealized Depreciation
Less than 12 months	297	596
12 months or longer	201	386
Total	$ 498	$ 982
Corporate bonds | Consumer goods and services
Number of Issues
Less than 12 months | issue	11	11
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 20,912	$ 21,059
12 months or longer	0	0
Total	20,912	21,059
Gross Unrealized Depreciation
Less than 12 months	269	295
12 months or longer	0	0
Total	$ 269	$ 295
Corporate bonds | Technology, media and telecommunications
Number of Issues
Less than 12 months | issue	13	16
12 months or longer | issue	3	3
Fair Value
Less than 12 months	$ 32,671	$ 41,230
12 months or longer	10,222	10,241
Total	42,893	51,471
Gross Unrealized Depreciation
Less than 12 months	397	516
12 months or longer	303	303
Total	$ 700	$ 819
Corporate bonds | Health care
Number of Issues
Less than 12 months | issue	3	9
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 8,472	$ 20,918
12 months or longer	0	0
Total	8,472	20,918
Gross Unrealized Depreciation
Less than 12 months	37	151
12 months or longer	0	0
Total	$ 37	$ 151
Corporate bonds | Financial services
Number of Issues
Less than 12 months | issue	24	37
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 51,512	$ 75,286
12 months or longer	0	0
Total	51,512	75,286
Gross Unrealized Depreciation
Less than 12 months	929	1,358
12 months or longer	0	0
Total	$ 929	$ 1,358
Mortgage-backed securities
Number of Issues
Less than 12 months | issue	19	16
12 months or longer | issue	4	5
Fair Value
Less than 12 months	$ 9,301	$ 9,611
12 months or longer	1,087	1,198
Total	10,388	10,809
Gross Unrealized Depreciation
Less than 12 months	195	187
12 months or longer	55	54
Total	$ 250	$ 241
Government national mortgage association
Number of Issues
Less than 12 months | issue	36	36
12 months or longer | issue	10	9
Fair Value
Less than 12 months	$ 82,052	$ 82,430
12 months or longer	17,273	13,603
Total	99,325	96,033
Gross Unrealized Depreciation
Less than 12 months	1,830	2,261
12 months or longer	621	505
Total	$ 2,451	$ 2,766
Federal home loan mortgage corporation
Number of Issues
Less than 12 months | issue	40	41
12 months or longer | issue	3	3
Fair Value
Less than 12 months	$ 100,351	$ 105,775
12 months or longer	4,969	5,141
Total	105,320	110,916
Gross Unrealized Depreciation
Less than 12 months	3,223	3,165
12 months or longer	230	241
Total	$ 3,453	$ 3,406
Federal national mortgage association
Number of Issues
Less than 12 months | issue	23	27
12 months or longer | issue	4	4
Fair Value
Less than 12 months	$ 39,294	$ 46,633
12 months or longer	4,180	4,341
Total	43,474	50,974
Gross Unrealized Depreciation
Less than 12 months	857	1,091
12 months or longer	233	243
Total	$ 1,090	$ 1,334
Asset-backed Securities [Member]
Number of Issues
Less than 12 months | issue	0	1
12 months or longer | issue	1	1
Fair Value
Less than 12 months	$ 0	$ 971
12 months or longer	2,647	2,559
Total	2,647	3,530
Gross Unrealized Depreciation
Less than 12 months	0	29
12 months or longer	186	253
Total	$ 186	$ 282
Common stocks | Energy
Number of Issues
Less than 12 months | issue	1	0
12 months or longer | issue	1	1
Fair Value
Less than 12 months	$ 190	$ 0
12 months or longer	158	163
Total	348	163
Gross Unrealized Depreciation
Less than 12 months	2	0
12 months or longer	27	22
Total	$ 29	$ 22
Common stocks | Industrials
Number of Issues
Less than 12 months | issue	0	0
12 months or longer | issue	6	6
Fair Value
Less than 12 months	$ 0	$ 0
12 months or longer	237	239
Total	237	239
Gross Unrealized Depreciation
Less than 12 months	0	0
12 months or longer	175	173
Total	$ 175	$ 173
Common stocks | Consumer goods and services
Number of Issues
Less than 12 months | issue	1	3
12 months or longer | issue	4	2
Fair Value
Less than 12 months	$ 22	$ 282
12 months or longer	269	15
Total	291	297
Gross Unrealized Depreciation
Less than 12 months	1	55
12 months or longer	63	3
Total	$ 64	$ 58
Common stocks | Technology, media and telecommunications
Number of Issues
Less than 12 months | issue	8	7
12 months or longer | issue	8	8
Fair Value
Less than 12 months	$ 93	$ 26
12 months or longer	22	33
Total	115	59
Gross Unrealized Depreciation
Less than 12 months	16	5
12 months or longer	43	33
Total	$ 59	$ 38
Common stocks | Financial services
Number of Issues
Less than 12 months | issue	3	3
12 months or longer | issue	2	2
Fair Value
Less than 12 months	$ 53	$ 53
12 months or longer	165	150
Total	218	203
Gross Unrealized Depreciation
Less than 12 months	2	3
12 months or longer	48	64
Total	$ 50	$ 67
Common stocks | Public utilities
Number of Issues
Less than 12 months | issue	0	0
12 months or longer | issue	3	3
Fair Value
Less than 12 months	$ 0	$ 0
12 months or longer	183	120
Total	183	120
Gross Unrealized Depreciation
Less than 12 months	0	0
12 months or longer	125	188
Total	$ 125	$ 188
Equity securities:
Number of Issues
Less than 12 months | issue	13	13
12 months or longer | issue	24	22
Fair Value
Less than 12 months	$ 358	$ 361
12 months or longer	1,034	720
Total	1,392	1,081
Gross Unrealized Depreciation
Less than 12 months	21	63
12 months or longer	481	483
Total	$ 502	$ 546