Investments - Available-For-Sale Securities in a Gross Unrealized Loss Position by Length of Time (Details) $ in Thousands	Dec. 31, 2022 USD ($) holding	Dec. 31, 2021 USD ($) holding
Investment grade
Fair value
Less than 12 months	$ 525,805	$ 330,697
12 months or longer	215,742	17,112
Total	741,547	347,809
Unrealized losses
Less than 12 months	31,904	3,801
12 months or longer	25,205	434
Total	$ 57,109	$ 4,235
No. of holdings | holding	761	366
Non-investment grade
Fair value
Less than 12 months	$ 82,849	$ 45,279
12 months or longer	14,607	6,110
Total	97,456	51,389
Unrealized losses
Less than 12 months	7,574	405
12 months or longer	2,688	415
Total	$ 10,262	$ 820
No. of holdings | holding	652	363
Corporate debt securities
Fair value
Less than 12 months	$ 397,511	$ 179,281
12 months or longer	121,094	12,494
Total	518,605	191,775
Unrealized losses
Less than 12 months	21,371	1,912
12 months or longer	14,440	583
Total	$ 35,811	$ 2,495
No. of holdings | holding	916	441
Collateralized debt obligations
Fair value
Less than 12 months	$ 44,823	$ 64,270
12 months or longer	55,335	9,370
Total	100,158	73,640
Unrealized losses
Less than 12 months	2,529	278
12 months or longer	2,675	60
Total	$ 5,204	$ 338
No. of holdings | holding	159	104
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 41,139	$ 28,001
12 months or longer	15,864	917
Total	57,003	28,918
Unrealized losses
Less than 12 months	5,124	595
12 months or longer	2,834	182
Total	$ 7,958	$ 777
No. of holdings | holding	131	61
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 109,499	$ 89,460
12 months or longer	31,465	441
Total	140,964	89,901
Unrealized losses
Less than 12 months	9,131	1,278
12 months or longer	6,938	24
Total	$ 16,069	$ 1,302
No. of holdings | holding	161	98
Other debt securities
Fair value
Less than 12 months	$ 15,682	$ 14,576
12 months or longer	6,591	0
Total	22,273	14,576
Unrealized losses
Less than 12 months	1,323	136
12 months or longer	1,006	0
Total	$ 2,329	$ 136
No. of holdings | holding	46	24
U.S. Treasury
Fair value
Less than 12 months		$ 388
12 months or longer		0
Total		388
Unrealized losses
Less than 12 months		7
12 months or longer		0
Total		$ 7
No. of holdings | holding		1
Available-for-sale securities
Fair value
Less than 12 months	$ 608,654	$ 375,976
12 months or longer	230,349	23,222
Total	839,003	399,198
Unrealized losses
Less than 12 months	39,478	4,206
12 months or longer	27,893	849
Total	$ 67,371	$ 5,055
No. of holdings | holding	1,413	729