Summary of Investments (Investments in Unrealized Loss Position) (Details) $ in Thousands	Sep. 30, 2016 USD ($) issue	Dec. 31, 2015 USD ($) issue
Number of Issues
Less than 12 months | issue	77	378
12 months or longer | issue	70	92
Fair Value
Less than 12 months	$ 184,388	$ 747,083
12 months or longer	74,873	140,558
Total	259,261	887,641
Gross Unrealized Depreciation
Less than 12 months	2,073	17,414
12 months or longer	2,933	10,677
Total	$ 5,006	$ 28,091
Fixed Maturities
Number of Issues
Less than 12 months | issue	68	350
12 months or longer | issue	42	79
Fair Value
Less than 12 months	$ 183,844	$ 743,274
12 months or longer	73,244	140,088
Total	257,088	883,362
Gross Unrealized Depreciation
Less than 12 months	2,031	16,962
12 months or longer	2,378	10,140
Total	$ 4,409	$ 27,102
States, municipalities and political subdivisions | Special revenue: | South
Number of Issues
Less than 12 months | issue	14	4
12 months or longer | issue	0	2
Fair Value
Less than 12 months	$ 37,375	$ 7,445
12 months or longer	0	1,851
Total	37,375	9,296
Gross Unrealized Depreciation
Less than 12 months	542	26
12 months or longer	0	1
Total	$ 542	$ 27
States, municipalities and political subdivisions | Special revenue: | Midwest
Number of Issues
Less than 12 months | issue	3	0
12 months or longer | issue	0	1
Fair Value
Less than 12 months	$ 6,008	$ 0
12 months or longer	0	2,494
Total	6,008	2,494
Gross Unrealized Depreciation
Less than 12 months	56	0
12 months or longer	0	30
Total	$ 56	$ 30
States, municipalities and political subdivisions | Special revenue: | West
Number of Issues
Less than 12 months | issue	9	4
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 19,130	$ 6,851
12 months or longer	0	0
Total	19,130	6,851
Gross Unrealized Depreciation
Less than 12 months	184	44
12 months or longer	0	0
Total	$ 184	$ 44
States, municipalities and political subdivisions | Special revenue: | Northeast
Number of Issues
Less than 12 months | issue	6	1
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 13,978	$ 4,755
12 months or longer	0	0
Total	13,978	4,755
Gross Unrealized Depreciation
Less than 12 months	98	212
12 months or longer	0	0
Total	$ 98	$ 212
States, municipalities and political subdivisions | General obligations: | South
Number of Issues
Less than 12 months | issue	7	3
12 months or longer | issue	0	8
Fair Value
Less than 12 months	$ 16,784	$ 4,805
12 months or longer	0	3,743
Total	16,784	8,548
Gross Unrealized Depreciation
Less than 12 months	314	55
12 months or longer	0	79
Total	$ 314	$ 134
States, municipalities and political subdivisions | General obligations: | Midwest
Number of Issues
Less than 12 months | issue	2	4
12 months or longer | issue	0	1
Fair Value
Less than 12 months	$ 9,909	$ 2,417
12 months or longer	0	528
Total	9,909	2,945
Gross Unrealized Depreciation
Less than 12 months	90	12
12 months or longer	0	6
Total	$ 90	$ 18
States, municipalities and political subdivisions | General obligations: | West
Number of Issues
Less than 12 months | issue	6	4
12 months or longer | issue	0	2
Fair Value
Less than 12 months	$ 16,353	$ 8,927
12 months or longer	0	2,274
Total	16,353	11,201
Gross Unrealized Depreciation
Less than 12 months	180	23
12 months or longer	0	44
Total	$ 180	$ 67
Foreign bonds
Number of Issues
Less than 12 months | issue	0	9
12 months or longer | issue	3	2
Fair Value
Less than 12 months	$ 0	$ 16,991
12 months or longer	6,929	4,036
Total	6,929	21,027
Gross Unrealized Depreciation
Less than 12 months	0	1,289
12 months or longer	894	1,168
Total	$ 894	$ 2,457
Public utilities
Number of Issues
Less than 12 months | issue	0	35
12 months or longer | issue	5	5
Fair Value
Less than 12 months	$ 0	$ 72,680
12 months or longer	3,097	2,840
Total	3,097	75,520
Gross Unrealized Depreciation
Less than 12 months	0	880
12 months or longer	83	371
Total	$ 83	$ 1,251
Corporate bonds | Energy
Number of Issues
Less than 12 months | issue	3	29
12 months or longer | issue	4	4
Fair Value
Less than 12 months	$ 5,345	$ 61,496
12 months or longer	9,209	7,991
Total	14,554	69,487
Gross Unrealized Depreciation
Less than 12 months	40	3,286
12 months or longer	147	1,427
Total	$ 187	$ 4,713
Corporate bonds | Industrials
Number of Issues
Less than 12 months | issue	0	38
12 months or longer | issue	5	3
Fair Value
Less than 12 months	$ 0	$ 78,588
12 months or longer	9,522	6,649
Total	9,522	85,237
Gross Unrealized Depreciation
Less than 12 months	0	3,631
12 months or longer	668	3,032
Total	$ 668	$ 6,663
Corporate bonds | Consumer goods and services
Number of Issues
Less than 12 months | issue		24
12 months or longer | issue		4
Fair Value
Less than 12 months		$ 64,661
12 months or longer		2,491
Total		67,152
Gross Unrealized Depreciation
Less than 12 months		770
12 months or longer		6
Total		$ 776
Corporate bonds | Technology, media and telecommunications
Number of Issues
Less than 12 months | issue	1	22
12 months or longer | issue	3	2
Fair Value
Less than 12 months	$ 2,198	$ 59,503
12 months or longer	10,403	8,940
Total	12,601	68,443
Gross Unrealized Depreciation
Less than 12 months	8	1,478
12 months or longer	160	525
Total	$ 168	$ 2,003
Corporate bonds | Financial services
Number of Issues
Less than 12 months | issue	0	49
12 months or longer | issue	2	0
Fair Value
Less than 12 months	$ 0	$ 92,814
12 months or longer	8,036	0
Total	8,036	92,814
Gross Unrealized Depreciation
Less than 12 months	0	1,143
12 months or longer	20	0
Total	$ 20	$ 1,143
Corporate bonds | Health care
Number of Issues
Less than 12 months | issue		18
12 months or longer | issue		2
Fair Value
Less than 12 months		$ 43,992
12 months or longer		3,737
Total		47,729
Gross Unrealized Depreciation
Less than 12 months		652
12 months or longer		139
Total		$ 791
Mortgage-backed securities
Number of Issues
Less than 12 months | issue	0	9
12 months or longer | issue	4	4
Fair Value
Less than 12 months	$ 0	$ 7,423
12 months or longer	1,262	183
Total	1,262	7,606
Gross Unrealized Depreciation
Less than 12 months	0	43
12 months or longer	8	8
Total	$ 8	$ 51
Government national mortgage association
Number of Issues
Less than 12 months | issue	5	17
12 months or longer | issue	9	14
Fair Value
Less than 12 months	$ 12,340	$ 29,769
12 months or longer	14,097	40,027
Total	26,437	69,796
Gross Unrealized Depreciation
Less than 12 months	88	437
12 months or longer	201	1,548
Total	$ 289	$ 1,985
Federal home loan mortgage corporation
Number of Issues
Less than 12 months | issue	7	20
12 months or longer | issue	3	6
Fair Value
Less than 12 months	$ 31,530	$ 35,343
12 months or longer	5,977	19,887
Total	37,507	55,230
Gross Unrealized Depreciation
Less than 12 months	175	644
12 months or longer	87	698
Total	$ 262	$ 1,342
Federal national mortgage association
Number of Issues
Less than 12 months | issue	1	15
12 months or longer | issue	4	11
Fair Value
Less than 12 months	$ 5,001	$ 32,800
12 months or longer	4,712	11,962
Total	9,713	44,762
Gross Unrealized Depreciation
Less than 12 months	21	524
12 months or longer	110	417
Total	$ 131	$ 941
Asset-backed Securities [Member]
Number of Issues
Less than 12 months | issue	1	1
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 2,571	$ 985
12 months or longer	0	0
Total	2,571	985
Gross Unrealized Depreciation
Less than 12 months	231	16
12 months or longer	0	0
Total	$ 231	$ 16
Common stocks | Energy
Number of Issues
Less than 12 months | issue	1	10
12 months or longer | issue	2	0
Fair Value
Less than 12 months	$ 150	$ 2,868
12 months or longer	339	0
Total	489	2,868
Gross Unrealized Depreciation
Less than 12 months	1	266
12 months or longer	38	0
Total	$ 39	$ 266
Common stocks | Industrials
Number of Issues
Less than 12 months | issue	0	3
12 months or longer | issue	6	5
Fair Value
Less than 12 months	$ 0	$ 177
12 months or longer	197	193
Total	197	370
Gross Unrealized Depreciation
Less than 12 months	0	44
12 months or longer	215	269
Total	$ 215	$ 313
Common stocks | Consumer goods and services
Number of Issues
Less than 12 months | issue	3	0
12 months or longer | issue	2	2
Fair Value
Less than 12 months	$ 299	$ 0
12 months or longer	14	14
Total	313	14
Gross Unrealized Depreciation
Less than 12 months	39	0
12 months or longer	4	3
Total	$ 43	$ 3
Common stocks | Technology, media and telecommunications
Number of Issues
Less than 12 months | issue	4	9
12 months or longer | issue	11	2
Fair Value
Less than 12 months	$ 15	$ 438
12 months or longer	519	12
Total	534	450
Gross Unrealized Depreciation
Less than 12 months	1	91
12 months or longer	36	14
Total	$ 37	$ 105
Common stocks | Financial services
Number of Issues
Less than 12 months | issue	1	6
12 months or longer | issue	4	1
Fair Value
Less than 12 months	$ 80	$ 326
12 months or longer	451	136
Total	531	462
Gross Unrealized Depreciation
Less than 12 months	1	51
12 months or longer	64	58
Total	$ 65	$ 109
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	109	115
Total	109	115
Gross Unrealized Depreciation
Less than 12 months	0	0
12 months or longer	198	193
Total	$ 198	$ 193
Equity securities:
Number of Issues
Less than 12 months | issue	9	28
12 months or longer | issue	28	13
Fair Value
Less than 12 months	$ 544	$ 3,809
12 months or longer	1,629	470
Total	2,173	4,279
Gross Unrealized Depreciation
Less than 12 months	42	452
12 months or longer	555	537
Total	$ 597	$ 989
U.S. Treasury
Number of Issues
Less than 12 months | issue	2	6
12 months or longer | issue	0	2
Fair Value
Less than 12 months	$ 1,625	$ 6,408
12 months or longer	0	1,634
Total	1,625	8,042
Gross Unrealized Depreciation
Less than 12 months	1	26
12 months or longer	0	12
Total	$ 1	$ 38
U.S. government agency
Number of Issues
Less than 12 months | issue	1	38
12 months or longer | issue	0	6
Fair Value
Less than 12 months	$ 3,697	$ 104,621
12 months or longer	0	18,821
Total	3,697	123,442
Gross Unrealized Depreciation
Less than 12 months	3	1,771
12 months or longer	0	629
Total	$ 3	$ 2,400