Summary of Investments (Investments in Unrealized Loss Position) (Details) $ in Thousands	Mar. 31, 2019 USD ($) issue	Dec. 31, 2018 USD ($) issue
Number of Issues
Less than 12 months (issues) | issue	18	118
12 months or longer (issues) | issue	196	291
Fair Value
Less than 12 months	$ 32,905	$ 207,671
12 months or longer	453,677	698,835
Total	486,582	906,506
Gross Unrealized Depreciation
Less than 12 months	545	3,067
12 months or longer	6,532	20,578
Total	$ 7,077	$ 23,645
U.S. Treasury
Number of Issues
Less than 12 months (issues) | issue	1	1
12 months or longer (issues) | issue	5	5
Fair Value
Less than 12 months	$ 8,006	$ 8,018
12 months or longer	14,672	14,645
Total	22,678	22,663
Gross Unrealized Depreciation
Less than 12 months	2	7
12 months or longer	141	213
Total	$ 143	$ 220
U.S. government agency
Number of Issues
Less than 12 months (issues) | issue	0	4
12 months or longer (issues) | issue	9	17
Fair Value
Less than 12 months	$ 0	$ 17,907
12 months or longer	44,960	80,696
Total	44,960	98,603
Gross Unrealized Depreciation
Less than 12 months	0	81
12 months or longer	405	1,668
Total	$ 405	$ 1,749
General obligations | Midwest
Number of Issues
Less than 12 months (issues) | issue	0	2
12 months or longer (issues) | issue	3	7
Fair Value
Less than 12 months	$ 0	$ 2,939
12 months or longer	13,089	23,749
Total	13,089	26,688
Gross Unrealized Depreciation
Less than 12 months	0	5
12 months or longer	98	680
Total	$ 98	$ 685
General obligations | Northeast
Number of Issues
Less than 12 months (issues) | issue		0
12 months or longer (issues) | issue		3
Fair Value
Less than 12 months		$ 0
12 months or longer		12,110
Total		12,110
Gross Unrealized Depreciation
Less than 12 months		0
12 months or longer		103
Total		$ 103
General obligations | South
Number of Issues
Less than 12 months (issues) | issue	0	1
12 months or longer (issues) | issue	8	22
Fair Value
Less than 12 months	$ 0	$ 778
12 months or longer	24,642	50,174
Total	24,642	50,952
Gross Unrealized Depreciation
Less than 12 months	0	2
12 months or longer	475	1,551
Total	$ 475	$ 1,553
General obligations | West
Number of Issues
Less than 12 months (issues) | issue	0	1
12 months or longer (issues) | issue	8	16
Fair Value
Less than 12 months	$ 0	$ 1,203
12 months or longer	24,221	48,499
Total	24,221	49,702
Gross Unrealized Depreciation
Less than 12 months	0	5
12 months or longer	305	1,170
Total	$ 305	$ 1,175
Special revenue | Midwest
Number of Issues
Less than 12 months (issues) | issue	0	4
12 months or longer (issues) | issue	8	19
Fair Value
Less than 12 months	$ 0	$ 3,892
12 months or longer	18,051	43,854
Total	18,051	47,746
Gross Unrealized Depreciation
Less than 12 months	0	8
12 months or longer	194	862
Total	$ 194	$ 870
Special revenue | Northeast
Number of Issues
Less than 12 months (issues) | issue	0	0
12 months or longer (issues) | issue	7	14
Fair Value
Less than 12 months	$ 0	$ 0
12 months or longer	19,711	37,629
Total	19,711	37,629
Gross Unrealized Depreciation
Less than 12 months	0	0
12 months or longer	406	1,241
Total	$ 406	$ 1,241
Special revenue | South
Number of Issues
Less than 12 months (issues) | issue	0	4
12 months or longer (issues) | issue	25	45
Fair Value
Less than 12 months	$ 0	$ 4,298
12 months or longer	61,235	107,016
Total	61,235	111,314
Gross Unrealized Depreciation
Less than 12 months	0	30
12 months or longer	1,101	3,678
Total	$ 1,101	$ 3,708
Special revenue | West
Number of Issues
Less than 12 months (issues) | issue	0	4
12 months or longer (issues) | issue	18	28
Fair Value
Less than 12 months	$ 0	$ 11,115
12 months or longer	50,929	69,667
Total	50,929	80,782
Gross Unrealized Depreciation
Less than 12 months	0	32
12 months or longer	537	2,171
Total	$ 537	$ 2,203
Foreign bonds
Number of Issues
Less than 12 months (issues) | issue		1
12 months or longer (issues) | issue		0
Fair Value
Less than 12 months		$ 2,984
12 months or longer		0
Total		2,984
Gross Unrealized Depreciation
Less than 12 months		13
12 months or longer		0
Total		$ 13
Public utilities
Number of Issues
Less than 12 months (issues) | issue	4	12
12 months or longer (issues) | issue	7	8
Fair Value
Less than 12 months	$ 9,755	$ 25,781
12 months or longer	16,101	17,253
Total	25,856	43,034
Gross Unrealized Depreciation
Less than 12 months	244	552
12 months or longer	200	471
Total	$ 444	$ 1,023
Corporate bonds | Energy
Number of Issues
Less than 12 months (issues) | issue	0	7
12 months or longer (issues) | issue	2	2
Fair Value
Less than 12 months	$ 0	$ 12,556
12 months or longer	4,224	4,099
Total	4,224	16,655
Gross Unrealized Depreciation
Less than 12 months	0	148
12 months or longer	41	156
Total	$ 41	$ 304
Corporate bonds | Industrials
Number of Issues
Less than 12 months (issues) | issue	1	9
12 months or longer (issues) | issue	4	4
Fair Value
Less than 12 months	$ 5,010	$ 21,970
12 months or longer	12,389	11,040
Total	17,399	33,010
Gross Unrealized Depreciation
Less than 12 months	17	397
12 months or longer	154	509
Total	$ 171	$ 906
Corporate bonds | Consumer goods and services
Number of Issues
Less than 12 months (issues) | issue	0	14
12 months or longer (issues) | issue	6	5
Fair Value
Less than 12 months	$ 0	$ 30,399
12 months or longer	12,851	9,554
Total	12,851	39,953
Gross Unrealized Depreciation
Less than 12 months	0	527
12 months or longer	209	292
Total	$ 209	$ 819
Corporate bonds | Health care
Number of Issues
Less than 12 months (issues) | issue	0	3
12 months or longer (issues) | issue	1	1
Fair Value
Less than 12 months	$ 0	$ 6,203
12 months or longer	351	345
Total	351	6,548
Gross Unrealized Depreciation
Less than 12 months	0	97
12 months or longer	2	8
Total	$ 2	$ 105
Corporate bonds | Technology, media and telecommunications
Number of Issues
Less than 12 months (issues) | issue	0	6
12 months or longer (issues) | issue	4	5
Fair Value
Less than 12 months	$ 0	$ 12,638
12 months or longer	8,929	9,619
Total	8,929	22,257
Gross Unrealized Depreciation
Less than 12 months	0	288
12 months or longer	138	390
Total	$ 138	$ 678
Corporate bonds | Financial services
Number of Issues
Less than 12 months (issues) | issue	1	13
12 months or longer (issues) | issue	11	13
Fair Value
Less than 12 months	$ 4,744	$ 30,177
12 months or longer	29,445	32,855
Total	34,189	63,032
Gross Unrealized Depreciation
Less than 12 months	256	650
12 months or longer	413	1,525
Total	$ 669	$ 2,175
Mortgage-backed securities
Number of Issues
Less than 12 months (issues) | issue	9	22
12 months or longer (issues) | issue	27	22
Fair Value
Less than 12 months	$ 549	$ 1,539
12 months or longer	4,548	4,166
Total	5,097	5,705
Gross Unrealized Depreciation
Less than 12 months	4	34
12 months or longer	130	198
Total	$ 134	$ 232
Asset-backed securities
Number of Issues
Less than 12 months (issues) | issue	0	1
12 months or longer (issues) | issue	1	0
Fair Value
Less than 12 months	$ 0	$ 2,829
12 months or longer	2,835	0
Total	2,835	2,829
Gross Unrealized Depreciation
Less than 12 months	0	113
12 months or longer	125	0
Total	$ 125	$ 113
Government National Mortgage Association (GNMA) Insured Loans | Collateralized mortgage obligations
Number of Issues
Less than 12 months (issues) | issue	1	2
12 months or longer (issues) | issue	20	22
Fair Value
Less than 12 months	$ 4,841	$ 3,797
12 months or longer	43,142	44,690
Total	47,983	48,487
Gross Unrealized Depreciation
Less than 12 months	22	55
12 months or longer	741	1,679
Total	$ 763	$ 1,734
Federal Home Loan Mortgage Corporation (FHLMC) Insured Loans | Collateralized mortgage obligations
Number of Issues
Less than 12 months (issues) | issue	1	3
12 months or longer (issues) | issue	11	18
Fair Value
Less than 12 months	$ 0	$ 4,541
12 months or longer	26,018	38,189
Total	26,018	42,730
Gross Unrealized Depreciation
Less than 12 months	0	20
12 months or longer	513	1,284
Total	$ 513	$ 1,304
Federal National Mortgage Association (FNMA) Insured Loans | Collateralized mortgage obligations
Number of Issues
Less than 12 months (issues) | issue	0	4
12 months or longer (issues) | issue	11	15
Fair Value
Less than 12 months	$ 0	$ 2,107
12 months or longer	21,334	38,986
Total	21,334	41,093
Gross Unrealized Depreciation
Less than 12 months	0	3
12 months or longer	204	729
Total	$ 204	$ 732