Investments - Available-for-sale Securities in a Gross Unrealized Loss Position by Length of Time (Details) $ in Thousands	Dec. 31, 2023 USD ($) holding	Dec. 31, 2022 USD ($) holding
Investment grade
Fair value
Less than 12 months	$ 87,774	$ 525,805
12 months or longer	517,090	215,742
Total	604,864	741,547
Unrealized losses
Less than 12 months	807	31,904
12 months or longer	32,511	25,205
Total	$ 33,318	$ 57,109
No. of holdings | holding	651	761
Non-investment grade
Fair value
Less than 12 months	$ 8,493	$ 82,849
12 months or longer	47,618	14,607
Total	56,111	97,456
Unrealized losses
Less than 12 months	270	7,574
12 months or longer	4,089	2,688
Total	$ 4,359	$ 10,262
No. of holdings | holding	385	652
Corporate debt securities
Fair value
Less than 12 months	$ 50,853	$ 397,511
12 months or longer	338,322	121,094
Total	389,175	518,605
Unrealized losses
Less than 12 months	546	21,371
12 months or longer	15,999	14,440
Total	$ 16,545	$ 35,811
No. of holdings | holding	590	916
Collateralized debt obligations
Fair value
Less than 12 months	$ 3,911	$ 44,823
12 months or longer	87,005	55,335
Total	90,916	100,158
Unrealized losses
Less than 12 months	15	2,529
12 months or longer	2,073	2,675
Total	$ 2,088	$ 5,204
No. of holdings | holding	142	159
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 9,148	$ 41,139
12 months or longer	30,145	15,864
Total	39,293	57,003
Unrealized losses
Less than 12 months	157	5,124
12 months or longer	4,552	2,834
Total	$ 4,709	$ 7,958
No. of holdings | holding	108	131
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 30,271	$ 109,499
12 months or longer	101,761	31,465
Total	132,032	140,964
Unrealized losses
Less than 12 months	297	9,131
12 months or longer	13,350	6,938
Total	$ 13,647	$ 16,069
No. of holdings | holding	164	161
Other debt securities
Fair value
Less than 12 months	$ 2,084	$ 15,682
12 months or longer	7,475	6,591
Total	9,559	22,273
Unrealized losses
Less than 12 months	62	1,323
12 months or longer	626	1,006
Total	$ 688	$ 2,329
No. of holdings | holding	32	46
Available-for-sale securities
Fair value
Less than 12 months	$ 96,267	$ 608,654
12 months or longer	564,708	230,349
Total	660,975	839,003
Unrealized losses
Less than 12 months	1,077	39,478
12 months or longer	36,600	27,893
Total	$ 37,677	$ 67,371
No. of holdings | holding	1,036	1,413