Summary of Investments - Investments in Unrealized Loss Position (Details) $ in Thousands	Sep. 30, 2023 USD ($) issue	Dec. 31, 2022 USD ($) issue
Number of Issues
Less than 12 months | issue	294	534
12 months or longer | issue	470	100
Fair Value
Less than 12 months	$ 678,865	$ 1,085,847
12 months or longer	942,364	225,276
Total	1,621,229	1,311,123
Gross Unrealized Depreciation
Less than 12 months	22,642	58,671
12 months or longer	133,570	55,252
Total	$ 156,212	$ 113,923
U.S. Treasury
Number of Issues
Less than 12 months | issue	5	4
12 months or longer | issue	6	4
Fair Value
Less than 12 months	$ 54,358	$ 6,656
12 months or longer	12,515	8,019
Total	66,873	14,675
Gross Unrealized Depreciation
Less than 12 months	390	212
12 months or longer	870	797
Total	$ 1,260	$ 1,009
U.S. government agency
Number of Issues
Less than 12 months | issue	3	24
12 months or longer | issue	25	3
Fair Value
Less than 12 months	$ 10,643	$ 70,158
12 months or longer	75,239	11,242
Total	85,882	81,400
Gross Unrealized Depreciation
Less than 12 months	336	5,606
12 months or longer	12,477	4,115
Total	$ 12,813	$ 9,721
General obligations | Midwest
Number of Issues
Less than 12 months | issue	30	16
12 months or longer | issue	3	0
Fair Value
Less than 12 months	$ 45,359	$ 29,089
12 months or longer	10,357	0
Total	55,716	29,089
Gross Unrealized Depreciation
Less than 12 months	1,360	263
12 months or longer	483	0
Total	$ 1,843	$ 263
General obligations | Northeast
Number of Issues
Less than 12 months | issue	4	4
12 months or longer | issue	1	0
Fair Value
Less than 12 months	$ 7,716	$ 8,576
12 months or longer	3,283	0
Total	10,999	8,576
Gross Unrealized Depreciation
Less than 12 months	214	73
12 months or longer	227	0
Total	$ 441	$ 73
General obligations | South
Number of Issues
Less than 12 months | issue	23	24
12 months or longer | issue	9	0
Fair Value
Less than 12 months	$ 37,383	$ 48,235
12 months or longer	15,751	0
Total	53,134	48,235
Gross Unrealized Depreciation
Less than 12 months	989	927
12 months or longer	1,212	0
Total	$ 2,201	$ 927
General obligations | West
Number of Issues
Less than 12 months | issue	26	27
12 months or longer | issue	8	0
Fair Value
Less than 12 months	$ 54,944	$ 62,652
12 months or longer	20,382	0
Total	75,326	62,652
Gross Unrealized Depreciation
Less than 12 months	1,498	711
12 months or longer	1,220	0
Total	$ 2,718	$ 711
Special revenue | Midwest
Number of Issues
Less than 12 months | issue	38	35
12 months or longer | issue	11	0
Fair Value
Less than 12 months	$ 71,273	$ 67,101
12 months or longer	22,054	0
Total	93,327	67,101
Gross Unrealized Depreciation
Less than 12 months	2,694	1,065
12 months or longer	1,633	0
Total	$ 4,327	$ 1,065
Special revenue | Northeast
Number of Issues
Less than 12 months | issue	11	14
12 months or longer | issue	8	0
Fair Value
Less than 12 months	$ 27,061	$ 37,484
12 months or longer	23,491	0
Total	50,552	37,484
Gross Unrealized Depreciation
Less than 12 months	956	1,148
12 months or longer	1,328	0
Total	$ 2,284	$ 1,148
Special revenue | South
Number of Issues
Less than 12 months | issue	42	58
12 months or longer | issue	33	1
Fair Value
Less than 12 months	$ 95,076	$ 126,388
12 months or longer	64,424	866
Total	159,500	127,254
Gross Unrealized Depreciation
Less than 12 months	3,480	3,124
12 months or longer	5,415	405
Total	$ 8,895	$ 3,529
Special revenue | West
Number of Issues
Less than 12 months | issue	39	39
12 months or longer | issue	19	0
Fair Value
Less than 12 months	$ 59,134	$ 83,622
12 months or longer	41,380	0
Total	100,514	83,622
Gross Unrealized Depreciation
Less than 12 months	2,017	1,658
12 months or longer	2,988	0
Total	$ 5,005	$ 1,658
Foreign bonds
Number of Issues
Less than 12 months | issue	0	9
12 months or longer | issue	9	5
Fair Value
Less than 12 months	$ 0	$ 21,377
12 months or longer	18,336	10,272
Total	18,336	31,649
Gross Unrealized Depreciation
Less than 12 months	0	1,861
12 months or longer	2,915	2,619
Total	$ 2,915	$ 4,480
Public utilities
Number of Issues
Less than 12 months | issue	7	45
12 months or longer | issue	49	9
Fair Value
Less than 12 months	$ 17,689	$ 101,867
12 months or longer	109,217	19,979
Total	126,906	121,846
Gross Unrealized Depreciation
Less than 12 months	621	8,737
12 months or longer	15,138	4,669
Total	$ 15,759	$ 13,406
Corporate bonds | Energy
Number of Issues
Less than 12 months | issue	4	15
12 months or longer | issue	15	1
Fair Value
Less than 12 months	$ 8,604	$ 28,612
12 months or longer	32,996	4,597
Total	41,600	33,209
Gross Unrealized Depreciation
Less than 12 months	294	1,930
12 months or longer	3,474	1,368
Total	$ 3,768	$ 3,298
Corporate bonds | Industrials
Number of Issues
Less than 12 months | issue	6	21
12 months or longer | issue	22	4
Fair Value
Less than 12 months	$ 14,842	$ 43,639
12 months or longer	47,010	7,049
Total	61,852	50,688
Gross Unrealized Depreciation
Less than 12 months	331	3,542
12 months or longer	7,374	2,012
Total	$ 7,705	$ 5,554
Corporate bonds | Consumer goods and services
Number of Issues
Less than 12 months | issue	7	28
12 months or longer | issue	28	7
Fair Value
Less than 12 months	$ 22,863	$ 69,320
12 months or longer	65,643	20,620
Total	88,506	89,940
Gross Unrealized Depreciation
Less than 12 months	976	4,440
12 months or longer	10,818	6,157
Total	$ 11,794	$ 10,597
Corporate bonds | Health care
Number of Issues
Less than 12 months | issue	3	5
12 months or longer | issue	11	6
Fair Value
Less than 12 months	$ 6,604	$ 9,829
12 months or longer	24,933	15,928
Total	31,537	25,757
Gross Unrealized Depreciation
Less than 12 months	223	487
12 months or longer	6,143	4,933
Total	$ 6,366	$ 5,420
Corporate bonds | Technology, media and telecommunications
Number of Issues
Less than 12 months | issue	5	23
12 months or longer | issue	28	5
Fair Value
Less than 12 months	$ 17,505	$ 49,970
12 months or longer	61,492	9,970
Total	78,997	59,940
Gross Unrealized Depreciation
Less than 12 months	865	3,279
12 months or longer	9,172	3,974
Total	$ 10,037	$ 7,253
Corporate bonds | Financial services
Number of Issues
Less than 12 months | issue	10	40
12 months or longer | issue	44	5
Fair Value
Less than 12 months	$ 24,655	$ 101,411
12 months or longer	108,507	11,236
Total	133,162	112,647
Gross Unrealized Depreciation
Less than 12 months	1,090	6,997
12 months or longer	11,225	2,208
Total	$ 12,315	$ 9,205
Mortgage-backed securities
Number of Issues
Less than 12 months | issue	4	38
12 months or longer | issue	48	12
Fair Value
Less than 12 months	$ 5,656	$ 7,909
12 months or longer	15,352	9,791
Total	21,008	17,700
Gross Unrealized Depreciation
Less than 12 months	195	1,056
12 months or longer	3,268	1,693
Total	$ 3,463	$ 2,749
Collateralized mortgage obligations | Government national mortgage association
Number of Issues
Less than 12 months | issue	18	14
12 months or longer | issue	40	7
Fair Value
Less than 12 months	$ 75,767	$ 24,146
12 months or longer	70,903	16,674
Total	146,670	40,820
Gross Unrealized Depreciation
Less than 12 months	1,169	1,281
12 months or longer	16,054	3,611
Total	$ 17,223	$ 4,892
Collateralized mortgage obligations | Federal home loan mortgage corporation
Number of Issues
Less than 12 months | issue	3	29
12 months or longer | issue	32	12
Fair Value
Less than 12 months	$ 7,655	$ 48,898
12 months or longer	64,194	35,650
Total	71,849	84,548
Gross Unrealized Depreciation
Less than 12 months	2,523	4,500
12 months or longer	14,193	8,791
Total	$ 16,716	$ 13,291
Collateralized mortgage obligations | Federal national mortgage association
Number of Issues
Less than 12 months | issue	6	21
12 months or longer | issue	20	19
Fair Value
Less than 12 months	$ 14,078	$ 35,456
12 months or longer	31,810	43,383
Total	45,888	78,839
Gross Unrealized Depreciation
Less than 12 months	421	5,629
12 months or longer	5,806	7,900
Total	$ 6,227	$ 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	3,095	0
Total	3,095	3,452
Gross Unrealized Depreciation
Less than 12 months	0	145
12 months or longer	137	0
Total	$ 137	$ 145