Summary of Investments - Investments in Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2023 USD ($) issue	Dec. 31, 2022 USD ($) issue
Number of Issues
Less than 12 months | issue	318	534
12 months or longer | issue	250	100
Fair Value
Less than 12 months	$ 672,151	$ 1,085,847
12 months or longer	529,681	225,276
Total	1,201,832	1,311,123
Gross Unrealized Depreciation
Less than 12 months	20,486	58,671
12 months or longer	76,675	55,252
Total	$ 97,161	$ 113,923
U.S. Treasury
Number of Issues
Less than 12 months | issue	0	4
12 months or longer | issue	6	4
Fair Value
Less than 12 months	$ 0	$ 6,656
12 months or longer	12,633	8,019
Total	12,633	14,675
Gross Unrealized Depreciation
Less than 12 months	0	212
12 months or longer	829	797
Total	$ 829	$ 1,009
U.S. government agency
Number of Issues
Less than 12 months | issue	14	24
12 months or longer | issue	12	3
Fair Value
Less than 12 months	$ 42,728	$ 70,158
12 months or longer	36,407	11,242
Total	79,135	81,400
Gross Unrealized Depreciation
Less than 12 months	2,554	5,606
12 months or longer	6,384	4,115
Total	$ 8,938	$ 9,721
General obligations | Midwest
Number of Issues
Less than 12 months | issue	8	16
12 months or longer | issue	1	0
Fair Value
Less than 12 months	$ 12,164	$ 29,089
12 months or longer	7,779	0
Total	19,943	29,089
Gross Unrealized Depreciation
Less than 12 months	96	263
12 months or longer	61	0
Total	$ 157	$ 263
General obligations | Northeast
Number of Issues
Less than 12 months | issue	1	4
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 3,499	$ 8,576
12 months or longer	0	0
Total	3,499	8,576
Gross Unrealized Depreciation
Less than 12 months	24	73
12 months or longer	0	0
Total	$ 24	$ 73
General obligations | South
Number of Issues
Less than 12 months | issue	12	24
12 months or longer | issue	4	0
Fair Value
Less than 12 months	$ 25,768	$ 48,235
12 months or longer	10,864	0
Total	36,632	48,235
Gross Unrealized Depreciation
Less than 12 months	351	927
12 months or longer	150	0
Total	$ 501	$ 927
General obligations | West
Number of Issues
Less than 12 months | issue	18	27
12 months or longer | issue	1	0
Fair Value
Less than 12 months	$ 39,985	$ 62,652
12 months or longer	7,248	0
Total	47,233	62,652
Gross Unrealized Depreciation
Less than 12 months	284	711
12 months or longer	105	0
Total	$ 389	$ 711
Special revenue | Midwest
Number of Issues
Less than 12 months | issue	17	35
12 months or longer | issue	2	0
Fair Value
Less than 12 months	$ 36,384	$ 67,101
12 months or longer	3,627	0
Total	40,011	67,101
Gross Unrealized Depreciation
Less than 12 months	312	1,065
12 months or longer	88	0
Total	$ 400	$ 1,065
Special revenue | Northeast
Number of Issues
Less than 12 months | issue	8	14
12 months or longer | issue	4	0
Fair Value
Less than 12 months	$ 23,746	$ 37,484
12 months or longer	12,001	0
Total	35,747	37,484
Gross Unrealized Depreciation
Less than 12 months	236	1,148
12 months or longer	477	0
Total	$ 713	$ 1,148
Special revenue | South
Number of Issues
Less than 12 months | issue	41	58
12 months or longer | issue	10	1
Fair Value
Less than 12 months	$ 85,197	$ 126,388
12 months or longer	18,098	866
Total	103,295	127,254
Gross Unrealized Depreciation
Less than 12 months	1,481	3,124
12 months or longer	798	405
Total	$ 2,279	$ 3,529
Special revenue | West
Number of Issues
Less than 12 months | issue	26	39
12 months or longer | issue	6	0
Fair Value
Less than 12 months	$ 53,685	$ 83,622
12 months or longer	11,776	0
Total	65,461	83,622
Gross Unrealized Depreciation
Less than 12 months	646	1,658
12 months or longer	185	0
Total	$ 831	$ 1,658
Foreign bonds
Number of Issues
Less than 12 months | issue	7	9
12 months or longer | issue	7	5
Fair Value
Less than 12 months	$ 18,236	$ 21,377
12 months or longer	16,065	10,272
Total	34,301	31,649
Gross Unrealized Depreciation
Less than 12 months	1,033	1,861
12 months or longer	2,785	2,619
Total	$ 3,818	$ 4,480
Public utilities
Number of Issues
Less than 12 months | issue	24	45
12 months or longer | issue	31	9
Fair Value
Less than 12 months	$ 57,437	$ 101,867
12 months or longer	65,964	19,979
Total	123,401	121,846
Gross Unrealized Depreciation
Less than 12 months	2,288	8,737
12 months or longer	9,227	4,669
Total	$ 11,515	$ 13,406
Corporate bonds | Energy
Number of Issues
Less than 12 months | issue	13	15
12 months or longer | issue	2	1
Fair Value
Less than 12 months	$ 27,485	$ 28,612
12 months or longer	6,941	4,597
Total	34,426	33,209
Gross Unrealized Depreciation
Less than 12 months	788	1,930
12 months or longer	1,400	1,368
Total	$ 2,188	$ 3,298
Corporate bonds | Industrials
Number of Issues
Less than 12 months | issue	10	21
12 months or longer | issue	12	4
Fair Value
Less than 12 months	$ 26,075	$ 43,639
12 months or longer	20,941	7,049
Total	47,016	50,688
Gross Unrealized Depreciation
Less than 12 months	626	3,542
12 months or longer	4,002	2,012
Total	$ 4,628	$ 5,554
Corporate bonds | Consumer goods and services
Number of Issues
Less than 12 months | issue	19	28
12 months or longer | issue	14	7
Fair Value
Less than 12 months	$ 52,538	$ 69,320
12 months or longer	33,182	20,620
Total	85,720	89,940
Gross Unrealized Depreciation
Less than 12 months	1,651	4,440
12 months or longer	7,106	6,157
Total	$ 8,757	$ 10,597
Corporate bonds | Health care
Number of Issues
Less than 12 months | issue	4	5
12 months or longer | issue	7	6
Fair Value
Less than 12 months	$ 9,101	$ 9,829
12 months or longer	17,455	15,928
Total	26,556	25,757
Gross Unrealized Depreciation
Less than 12 months	222	487
12 months or longer	4,365	4,933
Total	$ 4,587	$ 5,420
Corporate bonds | Technology, media and telecommunications
Number of Issues
Less than 12 months | issue	17	23
12 months or longer | issue	11	5
Fair Value
Less than 12 months	$ 36,541	$ 49,970
12 months or longer	23,737	9,970
Total	60,278	59,940
Gross Unrealized Depreciation
Less than 12 months	953	3,279
12 months or longer	4,916	3,974
Total	$ 5,869	$ 7,253
Corporate bonds | Financial services
Number of Issues
Less than 12 months | issue	30	40
12 months or longer | issue	16	5
Fair Value
Less than 12 months	$ 72,896	$ 101,411
12 months or longer	41,551	11,236
Total	114,447	112,647
Gross Unrealized Depreciation
Less than 12 months	3,847	6,997
12 months or longer	5,509	2,208
Total	$ 9,356	$ 9,205
Mortgage-backed securities
Number of Issues
Less than 12 months | issue	26	38
12 months or longer | issue	26	12
Fair Value
Less than 12 months	$ 7,058	$ 7,909
12 months or longer	16,475	9,791
Total	23,533	17,700
Gross Unrealized Depreciation
Less than 12 months	67	1,056
12 months or longer	2,358	1,693
Total	$ 2,425	$ 2,749
Collateralized mortgage obligations | Federal home loan mortgage corporation
Number of Issues
Less than 12 months | issue	9	29
12 months or longer | issue	33	12
Fair Value
Less than 12 months	$ 16,349	$ 48,898
12 months or longer	70,165	35,650
Total	86,514	84,548
Gross Unrealized Depreciation
Less than 12 months	673	4,500
12 months or longer	11,963	8,791
Total	$ 12,636	$ 13,291
Collateralized mortgage obligations | Federal national mortgage association
Number of Issues
Less than 12 months | issue	7	21
12 months or longer | issue	31	19
Fair Value
Less than 12 months	$ 11,325	$ 35,456
12 months or longer	67,104	43,383
Total	78,429	78,839
Gross Unrealized Depreciation
Less than 12 months	2,024	5,629
12 months or longer	9,960	7,900
Total	$ 11,984	$ 13,529
Collateralized mortgage obligations | Government national mortgage association
Number of Issues
Less than 12 months | issue	6	14
12 months or longer | issue	14	7
Fair Value
Less than 12 months	$ 10,479	$ 24,146
12 months or longer	29,668	16,674
Total	40,147	40,820
Gross Unrealized Depreciation
Less than 12 months	220	1,281
12 months or longer	4,007	3,611
Total	$ 4,227	$ 4,892
Asset-backed securities
Number of Issues
Less than 12 months | issue	1	1
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 3,475	$ 3,452
12 months or longer	0	0
Total	3,475	3,452
Gross Unrealized Depreciation
Less than 12 months	110	145
12 months or longer	0	0
Total	$ 110	$ 145