Investments - Available-For-Sale Securities in a Gross Unrealized Loss Position by Length of Time (Details) $ in Thousands	Mar. 31, 2023 USD ($) holding	Dec. 31, 2022 USD ($) holding
Investment grade
Fair value
Less than 12 months	$ 244,721	$ 525,805
12 months or longer	451,973	215,742
Total	696,694	741,547
Unrealized losses
Less than 12 months	6,966	31,904
12 months or longer	40,258	25,205
Total	$ 47,224	$ 57,109
No. of holdings | holding	729	761
Non-investment grade
Fair value
Less than 12 months	$ 37,330	$ 82,849
12 months or longer	47,628	14,607
Total	84,958	97,456
Unrealized losses
Less than 12 months	1,668	7,574
12 months or longer	5,994	2,688
Total	$ 7,662	$ 10,262
No. of holdings | holding	580	652
Corporate debt securities
Fair value
Less than 12 months	$ 193,828	$ 397,511
12 months or longer	299,344	121,094
Total	493,172	518,605
Unrealized losses
Less than 12 months	5,680	21,371
12 months or longer	23,692	14,440
Total	$ 29,372	$ 35,811
No. of holdings | holding	835	916
Collateralized debt obligations
Fair value
Less than 12 months	$ 26,435	$ 44,823
12 months or longer	72,742	55,335
Total	99,177	100,158
Unrealized losses
Less than 12 months	1,297	2,529
12 months or longer	3,357	2,675
Total	$ 4,654	$ 5,204
No. of holdings | holding	157	159
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 14,800	$ 41,139
12 months or longer	36,816	15,864
Total	51,616	57,003
Unrealized losses
Less than 12 months	274	5,124
12 months or longer	6,024	2,834
Total	$ 6,298	$ 7,958
No. of holdings | holding	127	131
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 40,561	$ 109,499
12 months or longer	82,489	31,465
Total	123,050	140,964
Unrealized losses
Less than 12 months	1,152	9,131
12 months or longer	12,228	6,938
Total	$ 13,380	$ 16,069
No. of holdings | holding	153	161
Other debt securities
Fair value
Less than 12 months	$ 6,427	$ 15,682
12 months or longer	8,210	6,591
Total	14,637	22,273
Unrealized losses
Less than 12 months	231	1,323
12 months or longer	951	1,006
Total	$ 1,182	$ 2,329
No. of holdings | holding	37	46
Available-for-sale securities
Fair value
Less than 12 months	$ 282,051	$ 608,654
12 months or longer	499,601	230,349
Total	781,652	839,003
Unrealized losses
Less than 12 months	8,634	39,478
12 months or longer	46,252	27,893
Total	$ 54,886	$ 67,371
No. of holdings | holding	1,309	1,413