Investments - Available-For-Sale Securities in a Gross Unrealized Loss Position by Length of Time (Details) $ in Thousands	Sep. 30, 2023 USD ($) holding	Dec. 31, 2022 USD ($) holding
Investment grade
Fair value
Less than 12 months	$ 241,402	$ 525,805
12 months or longer	523,719	215,742
Total	765,121	741,547
Unrealized losses
Less than 12 months	8,142	31,904
12 months or longer	48,245	25,205
Total	$ 56,387	$ 57,109
No. of holdings | holding	814	761
Non-investment grade
Fair value
Less than 12 months	$ 28,905	$ 82,849
12 months or longer	51,925	14,607
Total	80,830	97,456
Unrealized losses
Less than 12 months	729	7,574
12 months or longer	6,696	2,688
Total	$ 7,425	$ 10,262
No. of holdings | holding	549	652
Corporate debt securities
Fair value
Less than 12 months	$ 173,565	$ 397,511
12 months or longer	345,584	121,094
Total	519,149	518,605
Unrealized losses
Less than 12 months	4,529	21,371
12 months or longer	25,818	14,440
Total	$ 30,347	$ 35,811
No. of holdings | holding	832	916
Collateralized debt obligations
Fair value
Less than 12 months	$ 7,010	$ 44,823
12 months or longer	93,940	55,335
Total	100,950	100,158
Unrealized losses
Less than 12 months	39	2,529
12 months or longer	2,789	2,675
Total	$ 2,828	$ 5,204
No. of holdings | holding	154	159
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 33,208	$ 41,139
12 months or longer	30,692	15,864
Total	63,900	57,003
Unrealized losses
Less than 12 months	1,064	5,124
12 months or longer	5,485	2,834
Total	$ 6,549	$ 7,958
No. of holdings | holding	156	131
Residential mortgage- backed securities
Fair value
Less than 12 months	$ 48,919	$ 109,499
12 months or longer	97,676	31,465
Total	146,595	140,964
Unrealized losses
Less than 12 months	3,118	9,131
12 months or longer	20,028	6,938
Total	$ 23,146	$ 16,069
No. of holdings | holding	182	161
Other debt securities
Fair value
Less than 12 months	$ 7,605	$ 15,682
12 months or longer	7,752	6,591
Total	15,357	22,273
Unrealized losses
Less than 12 months	121	1,323
12 months or longer	821	1,006
Total	$ 942	$ 2,329
No. of holdings | holding	39	46
Available-for-sale securities
Fair value
Less than 12 months	$ 270,307	$ 608,654
12 months or longer	575,644	230,349
Total	845,951	839,003
Unrealized losses
Less than 12 months	8,871	39,478
12 months or longer	54,941	27,893
Total	$ 63,812	$ 67,371
No. of holdings | holding	1,363	1,413