Investments - Available-For-Sale Securities in a Gross Unrealized Loss Position by Length of Time (Details) $ in Thousands	Jun. 30, 2024 USD ($) holding	Dec. 31, 2023 USD ($) holding
Investment grade
Fair value
Less than 12 months	$ 185,665	$ 87,774
12 months or longer	399,444	517,090
Total	585,109	604,864
Unrealized losses
Less than 12 months	1,371	807
12 months or longer	30,785	32,511
Total	$ 32,156	$ 33,318
No. of holdings | holding	691	651
Non-investment grade
Fair value
Less than 12 months	$ 20,323	$ 8,493
12 months or longer	35,245	47,618
Total	55,568	56,111
Unrealized losses
Less than 12 months	341	270
12 months or longer	2,869	4,089
Total	$ 3,210	$ 4,359
No. of holdings | holding	393	385
Corporate debt securities
Fair value
Less than 12 months	$ 119,642	$ 50,853
12 months or longer	256,352	338,322
Total	375,994	389,175
Unrealized losses
Less than 12 months	909	546
12 months or longer	11,831	15,999
Total	$ 12,740	$ 16,545
No. of holdings | holding	611	590
Collateralized debt obligations
Fair value
Less than 12 months	$ 32,909	$ 3,911
12 months or longer	30,379	87,005
Total	63,288	90,916
Unrealized losses
Less than 12 months	64	15
12 months or longer	802	2,073
Total	$ 866	$ 2,088
No. of holdings | holding	116	142
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 33,653	$ 9,148
12 months or longer	29,248	30,145
Total	62,901	39,293
Unrealized losses
Less than 12 months	316	157
12 months or longer	3,820	4,552
Total	$ 4,136	$ 4,709
No. of holdings | holding	153	108
Residential mortgage- backed securities
Fair value
Less than 12 months	$ 12,704	$ 30,271
12 months or longer	112,051	101,761
Total	124,755	132,032
Unrealized losses
Less than 12 months	308	297
12 months or longer	16,682	13,350
Total	$ 16,990	$ 13,647
No. of holdings | holding	166	164
Other debt securities
Fair value
Less than 12 months	$ 6,055	$ 2,084
12 months or longer	6,659	7,475
Total	12,714	9,559
Unrealized losses
Less than 12 months	97	62
12 months or longer	519	626
Total	$ 616	$ 688
No. of holdings | holding	37	32
U.S. Treasury
Fair value
Less than 12 months	$ 1,025
12 months or longer	0
Total	1,025
Unrealized losses
Less than 12 months	18
12 months or longer	0
Total	$ 18
No. of holdings | holding	1
Available-for-sale securities
Fair value
Less than 12 months	$ 205,988	$ 96,267
12 months or longer	434,689	564,708
Total	640,677	660,975
Unrealized losses
Less than 12 months	1,712	1,077
12 months or longer	33,654	36,600
Total	$ 35,366	$ 37,677
No. of holdings | holding	1,084	1,036