Summary of Investments - Investments in Unrealized Loss Position (Details) $ in Thousands	Jun. 30, 2023 USD ($) issue	Dec. 31, 2022 USD ($) issue
Number of Issues
Less than 12 months | issue	344	534
12 months or longer | issue	344	100
Fair Value
Less than 12 months	$ 714,193	$ 1,085,847
12 months or longer	714,701	225,276
Total	1,428,894	1,311,123
Gross Unrealized Depreciation
Less than 12 months	20,008	58,671
12 months or longer	94,186	55,252
Total	$ 114,194	$ 113,923
U.S. Treasury
Number of Issues
Less than 12 months | issue	3	4
12 months or longer | issue	6	4
Fair Value
Less than 12 months	$ 9,082	$ 6,656
12 months or longer	12,482	8,019
Total	21,564	14,675
Gross Unrealized Depreciation
Less than 12 months	103	212
12 months or longer	942	797
Total	$ 1,045	$ 1,009
U.S. government agency
Number of Issues
Less than 12 months | issue	8	24
12 months or longer | issue	18	3
Fair Value
Less than 12 months	$ 23,264	$ 70,158
12 months or longer	57,981	11,242
Total	81,245	81,400
Gross Unrealized Depreciation
Less than 12 months	768	5,606
12 months or longer	8,753	4,115
Total	$ 9,521	$ 9,721
General obligations | Midwest
Number of Issues
Less than 12 months | issue	26	16
12 months or longer | issue	1	0
Fair Value
Less than 12 months	$ 38,149	$ 29,089
12 months or longer	7,728	0
Total	45,877	29,089
Gross Unrealized Depreciation
Less than 12 months	321	263
12 months or longer	90	0
Total	$ 411	$ 263
General obligations | Northeast
Number of Issues
Less than 12 months | issue	5	4
12 months or longer | issue	0	0
Fair Value
Less than 12 months	$ 11,398	$ 8,576
12 months or longer	0	0
Total	11,398	8,576
Gross Unrealized Depreciation
Less than 12 months	60	73
12 months or longer	0	0
Total	$ 60	$ 73
General obligations | South
Number of Issues
Less than 12 months | issue	22	24
12 months or longer | issue	5	0
Fair Value
Less than 12 months	$ 37,942	$ 48,235
12 months or longer	11,722	0
Total	49,664	48,235
Gross Unrealized Depreciation
Less than 12 months	403	927
12 months or longer	306	0
Total	$ 709	$ 927
General obligations | West
Number of Issues
Less than 12 months | issue	27	27
12 months or longer | issue	3	0
Fair Value
Less than 12 months	$ 61,813	$ 62,652
12 months or longer	11,055	0
Total	72,868	62,652
Gross Unrealized Depreciation
Less than 12 months	583	711
12 months or longer	212	0
Total	$ 795	$ 711
Special revenue | Midwest
Number of Issues
Less than 12 months | issue	29	35
12 months or longer | issue	8	0
Fair Value
Less than 12 months	$ 54,006	$ 67,101
12 months or longer	16,747	0
Total	70,753	67,101
Gross Unrealized Depreciation
Less than 12 months	444	1,065
12 months or longer	365	0
Total	$ 809	$ 1,065
Special revenue | Northeast
Number of Issues
Less than 12 months | issue	9	14
12 months or longer | issue	6	0
Fair Value
Less than 12 months	$ 22,914	$ 37,484
12 months or longer	17,412	0
Total	40,326	37,484
Gross Unrealized Depreciation
Less than 12 months	385	1,148
12 months or longer	697	0
Total	$ 1,082	$ 1,148
Special revenue | South
Number of Issues
Less than 12 months | issue	42	58
12 months or longer | issue	24	1
Fair Value
Less than 12 months	$ 90,055	$ 126,388
12 months or longer	52,927	866
Total	142,982	127,254
Gross Unrealized Depreciation
Less than 12 months	1,075	3,124
12 months or longer	2,178	405
Total	$ 3,253	$ 3,529
Special revenue | West
Number of Issues
Less than 12 months | issue	34	39
12 months or longer | issue	13	0
Fair Value
Less than 12 months	$ 50,810	$ 83,622
12 months or longer	36,389	0
Total	87,199	83,622
Gross Unrealized Depreciation
Less than 12 months	574	1,658
12 months or longer	975	0
Total	$ 1,549	$ 1,658
Foreign bonds
Number of Issues
Less than 12 months | issue	6	9
12 months or longer | issue	9	5
Fair Value
Less than 12 months	$ 17,051	$ 21,377
12 months or longer	19,722	10,272
Total	36,773	31,649
Gross Unrealized Depreciation
Less than 12 months	1,207	1,861
12 months or longer	3,148	2,619
Total	$ 4,355	$ 4,480
Public utilities
Number of Issues
Less than 12 months | issue	16	45
12 months or longer | issue	37	9
Fair Value
Less than 12 months	$ 42,949	$ 101,867
12 months or longer	78,805	19,979
Total	121,754	121,846
Gross Unrealized Depreciation
Less than 12 months	2,637	8,737
12 months or longer	10,781	4,669
Total	$ 13,418	$ 13,406
Corporate bonds | Energy
Number of Issues
Less than 12 months | issue	11	15
12 months or longer | issue	8	1
Fair Value
Less than 12 months	$ 22,087	$ 28,612
12 months or longer	19,256	4,597
Total	41,343	33,209
Gross Unrealized Depreciation
Less than 12 months	602	1,930
12 months or longer	2,503	1,368
Total	$ 3,105	$ 3,298
Corporate bonds | Industrials
Number of Issues
Less than 12 months | issue	11	21
12 months or longer | issue	13	4
Fair Value
Less than 12 months	$ 27,922	$ 43,639
12 months or longer	23,584	7,049
Total	51,506	50,688
Gross Unrealized Depreciation
Less than 12 months	919	3,542
12 months or longer	4,351	2,012
Total	$ 5,270	$ 5,554
Corporate bonds | Consumer goods and services
Number of Issues
Less than 12 months | issue	16	28
12 months or longer | issue	19	7
Fair Value
Less than 12 months	$ 44,704	$ 69,320
12 months or longer	45,817	20,620
Total	90,521	89,940
Gross Unrealized Depreciation
Less than 12 months	1,440	4,440
12 months or longer	8,407	6,157
Total	$ 9,847	$ 10,597
Corporate bonds | Health care
Number of Issues
Less than 12 months | issue	6	5
12 months or longer | issue	7	6
Fair Value
Less than 12 months	$ 13,920	$ 9,829
12 months or longer	17,048	15,928
Total	30,968	25,757
Gross Unrealized Depreciation
Less than 12 months	411	487
12 months or longer	4,734	4,933
Total	$ 5,145	$ 5,420
Corporate bonds | Technology, media and telecommunications
Number of Issues
Less than 12 months | issue	12	23
12 months or longer | issue	18	5
Fair Value
Less than 12 months	$ 27,692	$ 49,970
12 months or longer	37,672	9,970
Total	65,364	59,940
Gross Unrealized Depreciation
Less than 12 months	813	3,279
12 months or longer	5,919	3,974
Total	$ 6,732	$ 7,253
Corporate bonds | Financial services
Number of Issues
Less than 12 months | issue	22	40
12 months or longer | issue	28	5
Fair Value
Less than 12 months	$ 53,576	$ 101,411
12 months or longer	69,708	11,236
Total	123,284	112,647
Gross Unrealized Depreciation
Less than 12 months	3,054	6,997
12 months or longer	7,712	2,208
Total	$ 10,766	$ 9,205
Mortgage-backed securities
Number of Issues
Less than 12 months | issue	14	38
12 months or longer | issue	38	12
Fair Value
Less than 12 months	$ 6,658	$ 7,909
12 months or longer	16,015	9,791
Total	22,673	17,700
Gross Unrealized Depreciation
Less than 12 months	120	1,056
12 months or longer	2,546	1,693
Total	$ 2,666	$ 2,749
Collateralized mortgage obligations | Government national mortgage association
Number of Issues
Less than 12 months | issue	13	14
12 months or longer | issue	33	7
Fair Value
Less than 12 months	$ 28,290	$ 24,146
12 months or longer	67,170	16,674
Total	95,460	40,820
Gross Unrealized Depreciation
Less than 12 months	1,046	1,281
12 months or longer	13,323	3,611
Total	$ 14,369	$ 4,892
Collateralized mortgage obligations | Federal home loan mortgage corporation
Number of Issues
Less than 12 months | issue	5	29
12 months or longer | issue	33	12
Fair Value
Less than 12 months	$ 13,312	$ 48,898
12 months or longer	63,848	35,650
Total	77,160	84,548
Gross Unrealized Depreciation
Less than 12 months	2,534	4,500
12 months or longer	11,414	8,791
Total	$ 13,948	$ 13,291
Collateralized mortgage obligations | Federal national mortgage association
Number of Issues
Less than 12 months | issue	7	21
12 months or longer | issue	16	19
Fair Value
Less than 12 months	$ 16,599	$ 35,456
12 months or longer	28,324	43,383
Total	44,923	78,839
Gross Unrealized Depreciation
Less than 12 months	509	5,629
12 months or longer	4,704	7,900
Total	$ 5,213	$ 13,529
Asset-backed securities
Number of Issues
Less than 12 months | issue	0	1
12 months or longer | issue	1	0
Fair Value
Less than 12 months	$ 0	$ 3,452
12 months or longer	3,289	0
Total	3,289	3,452
Gross Unrealized Depreciation
Less than 12 months	0	145
12 months or longer	126	0
Total	$ 126	$ 145