Summary of Investments (Investments in Unrealized Loss Position) (Details) $ in Thousands	Jun. 30, 2017 USD ($) issue	Dec. 31, 2016 USD ($) issue
Number of Issues
Less than 12 months | issue	255	567
12 months or longer | issue	48	55
Fair Value
Less than 12 months	$ 674,868	$ 1,060,938
12 months or longer	82,999	53,798
Total	757,867	1,114,736
Gross Unrealized Depreciation
Less than 12 months	15,298	32,167
12 months or longer	3,764	2,598
Total	$ 19,062	$ 34,765
U.S. Treasury
Number of Issues
Less than 12 months | issue	4	9
12 months or longer | issue	2	0
Fair Value
Less than 12 months	$ 10,159	$ 10,800
12 months or longer	1,604	0
Total	11,763	10,800
Gross Unrealized Depreciation
Less than 12 months	78	108
12 months or longer	2	0
Total	$ 80	$ 108
U.S. government agency
Number of Issues
Less than 12 months | issue	8	10
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 33,669	$ 36,593
12 months or longer	0	0
Total	33,669	36,593
Gross Unrealized Depreciation
Less than 12 months	424	540
12 months or longer	0	0
Total	$ 424	$ 540
Fixed Maturities
Number of Issues
Less than 12 months | issue	248	554
12 months or longer | issue	33	33
Fair Value
Less than 12 months	$ 673,697	$ 1,060,577
12 months or longer	82,045	53,078
Total	755,742	1,113,655
Gross Unrealized Depreciation
Less than 12 months	15,144	32,104
12 months or longer	3,200	2,115
Total	$ 18,344	$ 34,219
States, municipalities and political subdivisions | General obligations: | Midwest
Number of Issues
Less than 12 months | issue	5	27
12 months or longer | issue	1	0
Fair Value
Less than 12 months	$ 17,233	$ 40,545
12 months or longer	5,254	0
Total	22,487	40,545
Gross Unrealized Depreciation
Less than 12 months	381	1,412
12 months or longer	174	0
Total	$ 555	$ 1,412
States, municipalities and political subdivisions | General obligations: | Northeast
Number of Issues
Less than 12 months | issue	1	9
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 3,574	$ 9,874
12 months or longer	0	0
Total	3,574	9,874
Gross Unrealized Depreciation
Less than 12 months	91	231
12 months or longer	0	0
Total	$ 91	$ 231
States, municipalities and political subdivisions | General obligations: | South
Number of Issues
Less than 12 months | issue	15	37
12 months or longer | issue	2	0
Fair Value
Less than 12 months	$ 36,447	$ 53,699
12 months or longer	9,271	0
Total	45,718	53,699
Gross Unrealized Depreciation
Less than 12 months	839	2,355
12 months or longer	511	0
Total	$ 1,350	$ 2,355
States, municipalities and political subdivisions | General obligations: | West
Number of Issues
Less than 12 months | issue	9	30
12 months or longer | issue	1	0
Fair Value
Less than 12 months	$ 29,069	$ 55,265
12 months or longer	2,029	0
Total	31,098	55,265
Gross Unrealized Depreciation
Less than 12 months	1,061	2,173
12 months or longer	104	0
Total	$ 1,165	$ 2,173
States, municipalities and political subdivisions | Special revenue: | Midwest
Number of Issues
Less than 12 months | issue	12	41
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 28,702	$ 62,937
12 months or longer	0	0
Total	28,702	62,937
Gross Unrealized Depreciation
Less than 12 months	678	1,433
12 months or longer	0	0
Total	$ 678	$ 1,433
States, municipalities and political subdivisions | Special revenue: | Northeast
Number of Issues
Less than 12 months | issue	16	22
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 48,096	$ 54,993
12 months or longer	0	0
Total	48,096	54,993
Gross Unrealized Depreciation
Less than 12 months	1,229	2,624
12 months or longer	0	0
Total	$ 1,229	$ 2,624
States, municipalities and political subdivisions | Special revenue: | South
Number of Issues
Less than 12 months | issue	33	79
12 months or longer | issue	4	0
Fair Value
Less than 12 months	$ 82,121	$ 152,979
12 months or longer	15,640	0
Total	97,761	152,979
Gross Unrealized Depreciation
Less than 12 months	2,461	6,791
12 months or longer	1,086	0
Total	$ 3,547	$ 6,791
States, municipalities and political subdivisions | Special revenue: | West
Number of Issues
Less than 12 months | issue	27	44
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 69,324	$ 81,676
12 months or longer	0	0
Total	69,324	81,676
Gross Unrealized Depreciation
Less than 12 months	2,377	4,052
12 months or longer	0	0
Total	$ 2,377	$ 4,052
Public utilities
Number of Issues
Less than 12 months | issue	8	20
12 months or longer | issue	1	2
Fair Value
Less than 12 months	$ 16,557	$ 38,511
12 months or longer	92	2,122
Total	16,649	40,633
Gross Unrealized Depreciation
Less than 12 months	235	423
12 months or longer	5	24
Total	$ 240	$ 447
Corporate bonds | Energy
Number of Issues
Less than 12 months | issue	5	8
12 months or longer | issue	0	3
Fair Value
Less than 12 months	$ 12,755	$ 15,938
12 months or longer	0	8,232
Total	12,755	24,170
Gross Unrealized Depreciation
Less than 12 months	200	313
12 months or longer	0	106
Total	$ 200	$ 419
Corporate bonds | Industrials
Number of Issues
Less than 12 months | issue	5	24
12 months or longer | issue	2	3
Fair Value
Less than 12 months	$ 9,004	$ 42,854
12 months or longer	4,803	5,641
Total	13,807	48,495
Gross Unrealized Depreciation
Less than 12 months	75	596
12 months or longer	144	386
Total	$ 219	$ 982
Corporate bonds | Consumer goods and services
Number of Issues
Less than 12 months | issue	8	11
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 13,832	$ 21,059
12 months or longer	0	0
Total	13,832	21,059
Gross Unrealized Depreciation
Less than 12 months	157	295
12 months or longer	0	0
Total	$ 157	$ 295
Corporate bonds | Technology, media and telecommunications
Number of Issues
Less than 12 months | issue	7	16
12 months or longer | issue	2	3
Fair Value
Less than 12 months	$ 20,121	$ 41,230
12 months or longer	10,584	10,241
Total	30,705	51,471
Gross Unrealized Depreciation
Less than 12 months	62	516
12 months or longer	113	303
Total	$ 175	$ 819
Corporate bonds | Health care
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 | Financial services
Number of Issues
Less than 12 months | issue	15	37
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 36,947	$ 75,286
12 months or longer	0	0
Total	36,947	75,286
Gross Unrealized Depreciation
Less than 12 months	526	1,358
12 months or longer	0	0
Total	$ 526	$ 1,358
Mortgage-backed securities
Number of Issues
Less than 12 months | issue	14	16
12 months or longer | issue	2	5
Fair Value
Less than 12 months	$ 8,927	$ 9,611
12 months or longer	996	1,198
Total	9,923	10,809
Gross Unrealized Depreciation
Less than 12 months	152	187
12 months or longer	43	54
Total	$ 195	$ 241
Government national mortgage association
Number of Issues
Less than 12 months | issue	18	36
12 months or longer | issue	8	9
Fair Value
Less than 12 months	$ 56,024	$ 82,430
12 months or longer	18,631	13,603
Total	74,655	96,033
Gross Unrealized Depreciation
Less than 12 months	945	2,261
12 months or longer	545	505
Total	$ 1,490	$ 2,766
Federal home loan mortgage corporation
Number of Issues
Less than 12 months | issue	26	41
12 months or longer | issue	3	3
Fair Value
Less than 12 months	$ 107,586	$ 105,775
12 months or longer	4,814	5,141
Total	112,400	110,916
Gross Unrealized Depreciation
Less than 12 months	2,693	3,165
12 months or longer	201	241
Total	$ 2,894	$ 3,406
Federal national mortgage association
Number of Issues
Less than 12 months | issue	12	27
12 months or longer | issue	5	4
Fair Value
Less than 12 months	$ 33,550	$ 46,633
12 months or longer	8,327	4,341
Total	41,877	50,974
Gross Unrealized Depreciation
Less than 12 months	480	1,091
12 months or longer	272	243
Total	$ 752	$ 1,334
Asset-backed Securities [Member]
Number of Issues
Less than 12 months | issue		1
12 months or longer | issue		1
Fair Value
Less than 12 months		$ 971
12 months or longer		2,559
Total		3,530
Gross Unrealized Depreciation
Less than 12 months		29
12 months or longer		253
Total		$ 282
Common stocks | Energy
Number of Issues
Less than 12 months | issue	2	0
12 months or longer | issue	1	1
Fair Value
Less than 12 months	$ 553	$ 0
12 months or longer	160	163
Total	713	163
Gross Unrealized Depreciation
Less than 12 months	95	0
12 months or longer	26	22
Total	$ 121	$ 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	231	239
Total	231	239
Gross Unrealized Depreciation
Less than 12 months	0	0
12 months or longer	181	173
Total	$ 181	$ 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	215	15
Total	237	297
Gross Unrealized Depreciation
Less than 12 months	1	55
12 months or longer	117	3
Total	$ 118	$ 58
Common stocks | Technology, media and telecommunications
Number of Issues
Less than 12 months | issue	3	7
12 months or longer | issue	1	8
Fair Value
Less than 12 months	$ 544	$ 26
12 months or longer	14	33
Total	558	59
Gross Unrealized Depreciation
Less than 12 months	55	5
12 months or longer	52	33
Total	$ 107	$ 38
Common stocks | Financial services
Number of Issues
Less than 12 months | issue	1	3
12 months or longer | issue	2	2
Fair Value
Less than 12 months	$ 52	$ 53
12 months or longer	166	150
Total	218	203
Gross Unrealized Depreciation
Less than 12 months	3	3
12 months or longer	48	64
Total	$ 51	$ 67
Common stocks | Public utilities
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	168	120
Total	168	120
Gross Unrealized Depreciation
Less than 12 months	0	0
12 months or longer	140	188
Total	$ 140	$ 188
Equity securities:
Number of Issues
Less than 12 months | issue	7	13
12 months or longer | issue	15	22
Fair Value
Less than 12 months	$ 1,171	$ 361
12 months or longer	954	720
Total	2,125	1,081
Gross Unrealized Depreciation
Less than 12 months	154	63
12 months or longer	564	483
Total	$ 718	$ 546