Investments - Available-For-Sale Securities in a Gross Unrealized Loss Position by Length of Time (Details) $ in Thousands	Mar. 31, 2024 USD ($) holding	Dec. 31, 2023 USD ($) holding
Investment grade
Fair value
Less than 12 months	$ 132,559	$ 87,774
12 months or longer	465,791	517,090
Total	598,350	604,864
Unrealized losses
Less than 12 months	1,507	807
12 months or longer	32,547	32,511
Total	$ 34,054	$ 33,318
No. of holdings | holding	671	651
Non-investment grade
Fair value
Less than 12 months	$ 14,962	$ 8,493
12 months or longer	41,281	47,618
Total	56,243	56,111
Unrealized losses
Less than 12 months	256	270
12 months or longer	3,564	4,089
Total	$ 3,820	$ 4,359
No. of holdings | holding	407	385
Corporate debt securities
Fair value
Less than 12 months	$ 72,273	$ 50,853
12 months or longer	318,640	338,322
Total	390,913	389,175
Unrealized losses
Less than 12 months	728	546
12 months or longer	14,637	15,999
Total	$ 15,365	$ 16,545
No. of holdings | holding	615	590
Collateralized debt obligations
Fair value
Less than 12 months	$ 25,013	$ 3,911
12 months or longer	52,090	87,005
Total	77,103	90,916
Unrealized losses
Less than 12 months	52	15
12 months or longer	1,327	2,073
Total	$ 1,379	$ 2,088
No. of holdings | holding	136	142
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 16,051	$ 9,148
12 months or longer	28,294	30,145
Total	44,345	39,293
Unrealized losses
Less than 12 months	167	157
12 months or longer	4,139	4,552
Total	$ 4,306	$ 4,709
No. of holdings | holding	121	108
Residential mortgage- backed securities
Fair value
Less than 12 months	$ 29,413	$ 30,271
12 months or longer	100,997	101,761
Total	130,410	132,032
Unrealized losses
Less than 12 months	754	297
12 months or longer	15,429	13,350
Total	$ 16,183	$ 13,647
No. of holdings | holding	169	164
Other debt securities
Fair value
Less than 12 months	$ 3,732	$ 2,084
12 months or longer	7,051	7,475
Total	10,783	9,559
Unrealized losses
Less than 12 months	57	62
12 months or longer	579	626
Total	$ 636	$ 688
No. of holdings | holding	36	32
U.S. Treasury
Fair value
Less than 12 months	$ 1,039
12 months or longer	0
Total	1,039
Unrealized losses
Less than 12 months	5
12 months or longer	0
Total	$ 5
No. of holdings | holding	1
Available-for-sale securities
Fair value
Less than 12 months	$ 147,521	$ 96,267
12 months or longer	507,072	564,708
Total	654,593	660,975
Unrealized losses
Less than 12 months	1,763	1,077
12 months or longer	36,111	36,600
Total	$ 37,874	$ 37,677
No. of holdings | holding	1,078	1,036