Summary of Investments - Investments in Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2022 USD ($) issue	Dec. 31, 2021 USD ($) issue
Number of Issues
Less than 12 months | issue	660	97
12 months or longer | issue	70	15
Fair Value
Less than 12 months	$ 1,322,751	$ 304,080
12 months or longer	169,740	43,871
Total	1,492,491	347,951
Gross Unrealized Depreciation
Less than 12 months	93,717	7,283
12 months or longer	41,341	1,366
Total	$ 135,058	$ 8,649
U.S. Treasury
Number of Issues
Less than 12 months | issue	4	6
12 months or longer | issue	4	1
Fair Value
Less than 12 months	$ 6,472	$ 32,166
12 months or longer	7,982	2,837
Total	14,454	35,003
Gross Unrealized Depreciation
Less than 12 months	216	630
12 months or longer	869	88
Total	$ 1,085	$ 718
U.S. government agency
Number of Issues
Less than 12 months | issue	24	3
12 months or longer | issue	3	0
Fair Value
Less than 12 months	$ 69,948	$ 15,023
12 months or longer	11,417	0
Total	81,365	15,023
Gross Unrealized Depreciation
Less than 12 months	5,812	562
12 months or longer	3,997	0
Total	$ 9,809	$ 562
General obligations | Midwest
Number of Issues
Less than 12 months | issue	34
12 months or longer | issue	0
Fair Value
Less than 12 months	$ 58,949
12 months or longer	0
Total	58,949
Gross Unrealized Depreciation
Less than 12 months	1,357
12 months or longer	0
Total	$ 1,357
General obligations | Northeast
Number of Issues
Less than 12 months | issue	8
12 months or longer | issue	0
Fair Value
Less than 12 months	$ 16,807
12 months or longer	0
Total	16,807
Gross Unrealized Depreciation
Less than 12 months	298
12 months or longer	0
Total	$ 298
General obligations | South
Number of Issues
Less than 12 months | issue	33
12 months or longer | issue	0
Fair Value
Less than 12 months	$ 61,969
12 months or longer	0
Total	61,969
Gross Unrealized Depreciation
Less than 12 months	2,198
12 months or longer	0
Total	$ 2,198
General obligations | West
Number of Issues
Less than 12 months | issue	36
12 months or longer | issue	0
Fair Value
Less than 12 months	$ 82,240
12 months or longer	0
Total	82,240
Gross Unrealized Depreciation
Less than 12 months	2,230
12 months or longer	0
Total	$ 2,230
Special revenue | Midwest
Number of Issues
Less than 12 months | issue	47
12 months or longer | issue	0
Fair Value
Less than 12 months	$ 93,081
12 months or longer	0
Total	93,081
Gross Unrealized Depreciation
Less than 12 months	3,086
12 months or longer	0
Total	$ 3,086
Special revenue | Northeast
Number of Issues
Less than 12 months | issue	21
12 months or longer | issue	0
Fair Value
Less than 12 months	$ 53,143
12 months or longer	0
Total	53,143
Gross Unrealized Depreciation
Less than 12 months	2,281
12 months or longer	0
Total	$ 2,281
Special revenue | South
Number of Issues
Less than 12 months | issue	76
12 months or longer | issue	1
Fair Value
Less than 12 months	$ 177,218
12 months or longer	845
Total	178,063
Gross Unrealized Depreciation
Less than 12 months	7,200
12 months or longer	426
Total	$ 7,626
Special revenue | West
Number of Issues
Less than 12 months | issue	54
12 months or longer | issue	0
Fair Value
Less than 12 months	$ 100,516
12 months or longer	0
Total	100,516
Gross Unrealized Depreciation
Less than 12 months	4,528
12 months or longer	0
Total	$ 4,528
States, municipalities and political subdivisions | South
Number of Issues
Less than 12 months | issue		1
12 months or longer | issue		0
Fair Value
Less than 12 months		$ 1,195
12 months or longer		0
Total		1,195
Gross Unrealized Depreciation
Less than 12 months		78
12 months or longer		0
Total		$ 78
Foreign bonds
Number of Issues
Less than 12 months | issue	11	4
12 months or longer | issue	3	1
Fair Value
Less than 12 months	$ 25,778	$ 10,731
12 months or longer	5,357	1,952
Total	31,135	12,683
Gross Unrealized Depreciation
Less than 12 months	3,360	147
12 months or longer	1,633	50
Total	$ 4,993	$ 197
Public utilities
Number of Issues
Less than 12 months | issue	47	9
12 months or longer | issue	7	0
Fair Value
Less than 12 months	$ 104,282	$ 24,238
12 months or longer	15,494	0
Total	119,776	24,238
Gross Unrealized Depreciation
Less than 12 months	11,490	481
12 months or longer	4,070	0
Total	$ 15,560	$ 481
Corporate bonds | Energy
Number of Issues
Less than 12 months | issue	16	1
12 months or longer | issue	1	0
Fair Value
Less than 12 months	$ 32,464	$ 5,881
12 months or longer	2,260	0
Total	34,724	5,881
Gross Unrealized Depreciation
Less than 12 months	3,048	81
12 months or longer	746	0
Total	$ 3,794	$ 81
Corporate bonds | Industrials
Number of Issues
Less than 12 months | issue	23	4
12 months or longer | issue	1	0
Fair Value
Less than 12 months	$ 47,467	$ 8,902
12 months or longer	2,246	0
Total	49,713	8,902
Gross Unrealized Depreciation
Less than 12 months	5,794	162
12 months or longer	743	0
Total	$ 6,537	$ 162
Corporate bonds | Consumer goods and services
Number of Issues
Less than 12 months | issue	32	10
12 months or longer | issue	7	0
Fair Value
Less than 12 months	$ 80,971	$ 26,367
12 months or longer	18,406	0
Total	99,377	26,367
Gross Unrealized Depreciation
Less than 12 months	9,318	611
12 months or longer	7,498	0
Total	$ 16,816	$ 611
Corporate bonds | Health care
Number of Issues
Less than 12 months | issue	7	3
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 14,008	$ 20,550
12 months or longer	0	0
Total	14,008	20,550
Gross Unrealized Depreciation
Less than 12 months	1,632	461
12 months or longer	0	0
Total	$ 1,632	$ 461
Corporate bonds | Technology, media and telecommunications
Number of Issues
Less than 12 months | issue	24	4
12 months or longer | issue	3	1
Fair Value
Less than 12 months	$ 51,188	$ 11,204
12 months or longer	5,551	1,906
Total	56,739	13,110
Gross Unrealized Depreciation
Less than 12 months	5,163	739
12 months or longer	3,345	127
Total	$ 8,508	$ 866
Corporate bonds | Financial services
Number of Issues
Less than 12 months | issue	45	5
12 months or longer | issue	3	0
Fair Value
Less than 12 months	$ 111,766	$ 13,320
12 months or longer	4,302	0
Total	116,068	13,320
Gross Unrealized Depreciation
Less than 12 months	9,642	131
12 months or longer	1,205	0
Total	$ 10,847	$ 131
Mortgage-backed securities
Number of Issues
Less than 12 months | issue	44	12
12 months or longer | issue	6	0
Fair Value
Less than 12 months	$ 8,247	$ 13,740
12 months or longer	9,806	0
Total	18,053	13,740
Gross Unrealized Depreciation
Less than 12 months	1,162	229
12 months or longer	1,847	0
Total	$ 3,009	$ 229
Collateralized mortgage obligations | Federal home loan mortgage corporation
Number of Issues
Less than 12 months | issue	25	11
12 months or longer | issue	17	1
Fair Value
Less than 12 months	$ 41,694	$ 48,256
12 months or longer	42,935	1,032
Total	84,629	49,288
Gross Unrealized Depreciation
Less than 12 months	4,755	1,752
12 months or longer	6,071	20
Total	$ 10,826	$ 1,772
Collateralized mortgage obligations | Federal national mortgage association
Number of Issues
Less than 12 months | issue	15	18
12 months or longer | issue	7	7
Fair Value
Less than 12 months	$ 24,997	$ 50,701
12 months or longer	17,495	30,847
Total	42,492	81,548
Gross Unrealized Depreciation
Less than 12 months	1,237	698
12 months or longer	3,590	960
Total	$ 4,827	$ 1,658
Collateralized mortgage obligations | Government national mortgage association
Number of Issues
Less than 12 months | issue	33	6
12 months or longer | issue	7	4
Fair Value
Less than 12 months	$ 56,120	$ 21,806
12 months or longer	25,644	5,297
Total	81,764	27,103
Gross Unrealized Depreciation
Less than 12 months	7,726	521
12 months or longer	5,301	121
Total	$ 13,027	$ 642
Asset-backed securities
Number of Issues
Less than 12 months | issue	1
12 months or longer | issue	0
Fair Value
Less than 12 months	$ 3,426
12 months or longer	0
Total	3,426
Gross Unrealized Depreciation
Less than 12 months	184
12 months or longer	0
Total	$ 184