Investments - Available-For-Sale Securities in a Gross Unrealized Loss Position by Length of Time (Details) $ in Thousands	Jun. 30, 2023 USD ($) holding	Dec. 31, 2022 USD ($) holding
Investment grade
Fair value
Less than 12 months	$ 275,990	$ 525,805
12 months or longer	482,717	215,742
Total	758,707	741,547
Unrealized losses
Less than 12 months	6,914	31,904
12 months or longer	42,549	25,205
Total	$ 49,463	$ 57,109
No. of holdings | holding	790	761
Non-investment grade
Fair value
Less than 12 months	$ 25,859	$ 82,849
12 months or longer	56,099	14,607
Total	81,958	97,456
Unrealized losses
Less than 12 months	646	7,574
12 months or longer	6,537	2,688
Total	$ 7,183	$ 10,262
No. of holdings | holding	550	652
Corporate debt securities
Fair value
Less than 12 months	$ 183,248	$ 397,511
12 months or longer	328,904	121,094
Total	512,152	518,605
Unrealized losses
Less than 12 months	4,632	21,371
12 months or longer	25,510	14,440
Total	$ 30,142	$ 35,811
No. of holdings | holding	817	916
Collateralized debt obligations
Fair value
Less than 12 months	$ 9,239	$ 44,823
12 months or longer	92,447	55,335
Total	101,686	100,158
Unrealized losses
Less than 12 months	102	2,529
12 months or longer	3,726	2,675
Total	$ 3,828	$ 5,204
No. of holdings | holding	157	159
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 26,338	$ 41,139
12 months or longer	32,703	15,864
Total	59,041	57,003
Unrealized losses
Less than 12 months	352	5,124
12 months or longer	5,933	2,834
Total	$ 6,285	$ 7,958
No. of holdings | holding	153	131
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 73,239	$ 109,499
12 months or longer	78,972	31,465
Total	152,211	140,964
Unrealized losses
Less than 12 months	2,212	9,131
12 months or longer	13,294	6,938
Total	$ 15,506	$ 16,069
No. of holdings | holding	177	161
Other debt securities
Fair value
Less than 12 months	$ 9,785	$ 15,682
12 months or longer	5,790	6,591
Total	15,575	22,273
Unrealized losses
Less than 12 months	262	1,323
12 months or longer	623	1,006
Total	$ 885	$ 2,329
No. of holdings | holding	36	46
Available-for-sale securities
Fair value
Less than 12 months	$ 301,849	$ 608,654
12 months or longer	538,816	230,349
Total	840,665	839,003
Unrealized losses
Less than 12 months	7,560	39,478
12 months or longer	49,086	27,893
Total	$ 56,646	$ 67,371
No. of holdings | holding	1,340	1,413