Investments - Available-For-Sale Securities in a Gross Unrealized Loss Position (Details) $ in Thousands	Mar. 31, 2022 USD ($) holding	Dec. 31, 2021 USD ($) holding
Investment grade
Fair value
Less than 12 months	$ 614,243	$ 330,697
12 months or longer	45,917	17,112
Total	660,160	347,809
Unrealized losses
Less than 12 months	21,202	3,801
12 months or longer	3,275	434
Total	$ 24,477	$ 4,235
No. of holdings | holding	634	366
Non-investment grade
Fair value
Less than 12 months	$ 95,519	$ 45,279
12 months or longer	8,324	6,110
Total	103,843	51,389
Unrealized losses
Less than 12 months	3,152	405
12 months or longer	330	415
Total	$ 3,482	$ 820
No. of holdings | holding	692	363
Corporate debt securities
Fair value
Less than 12 months	$ 421,303	$ 179,281
12 months or longer	30,331	12,494
Total	451,634	191,775
Unrealized losses
Less than 12 months	13,257	1,912
12 months or longer	2,095	583
Total	$ 15,352	$ 2,495
No. of holdings | holding	892	441
Collateralized debt obligations
Fair value
Less than 12 months	$ 74,277	$ 64,270
12 months or longer	14,477	9,370
Total	88,754	73,640
Unrealized losses
Less than 12 months	771	278
12 months or longer	208	60
Total	$ 979	$ 338
No. of holdings | holding	135	104
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 67,202	$ 28,001
12 months or longer	2,894	917
Total	70,096	28,918
Unrealized losses
Less than 12 months	3,027	595
12 months or longer	600	182
Total	$ 3,627	$ 777
No. of holdings | holding	125	61
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 129,659	$ 89,460
12 months or longer	6,539	441
Total	136,198	89,901
Unrealized losses
Less than 12 months	6,348	1,278
12 months or longer	702	24
Total	$ 7,050	$ 1,302
No. of holdings | holding	137	98
Other debt securities
Fair value
Less than 12 months	$ 16,382	$ 14,576
12 months or longer	0	0
Total	16,382	14,576
Unrealized losses
Less than 12 months	893	136
12 months or longer	0	0
Total	$ 893	$ 136
No. of holdings | holding	35	24
U.S. Treasury
Fair value
Less than 12 months	$ 939	$ 388
12 months or longer	0	0
Total	939	388
Unrealized losses
Less than 12 months	58	7
12 months or longer	0	0
Total	$ 58	$ 7
No. of holdings | holding	2	1
Available-for-sale securities
Fair value
Less than 12 months	$ 709,762	$ 375,976
12 months or longer	54,241	23,222
Total	764,003	399,198
Unrealized losses
Less than 12 months	24,354	4,206
12 months or longer	3,605	849
Total	$ 27,959	$ 5,055
No. of holdings | holding	1,326	729