Investments - Schedule of Available-For-Sale Securities in a Gross Unrealized Loss Position by Length of Time (Details) $ in Thousands	Jun. 30, 2022 USD ($) holding	Dec. 31, 2021 USD ($) holding
Investment grade
Fair value
Less than 12 months	$ 703,654	$ 330,697
12 months or longer	54,745	17,112
Total	758,399	347,809
Unrealized losses
Less than 12 months	41,032	3,801
12 months or longer	5,033	434
Total	$ 46,065	$ 4,235
No. of holdings | holding	719	366
Non-investment grade
Fair value
Less than 12 months	$ 105,199	$ 45,279
12 months or longer	5,576	6,110
Total	110,775	51,389
Unrealized losses
Less than 12 months	11,499	405
12 months or longer	680	415
Total	$ 12,179	$ 820
No. of holdings | holding	747	363
Corporate debt securities
Fair value
Less than 12 months	$ 518,575	$ 179,281
12 months or longer	26,993	12,494
Total	545,568	191,775
Unrealized losses
Less than 12 months	30,975	1,912
12 months or longer	2,946	583
Total	$ 33,921	$ 2,495
No. of holdings | holding	1,006	441
Collateralized debt obligations
Fair value
Less than 12 months	$ 73,825	$ 64,270
12 months or longer	22,805	9,370
Total	96,630	73,640
Unrealized losses
Less than 12 months	2,684	278
12 months or longer	1,017	60
Total	$ 3,701	$ 338
No. of holdings | holding	148	104
Commercial mortgage-backed securities
Fair value
Less than 12 months	$ 60,844	$ 28,001
12 months or longer	3,827	917
Total	64,671	28,918
Unrealized losses
Less than 12 months	5,394	595
12 months or longer	584	182
Total	$ 5,978	$ 777
No. of holdings | holding	121	61
Residential mortgage-backed securities
Fair value
Less than 12 months	$ 127,928	$ 89,460
12 months or longer	6,696	441
Total	134,624	89,901
Unrealized losses
Less than 12 months	11,922	1,278
12 months or longer	1,166	24
Total	$ 13,088	$ 1,302
No. of holdings | holding	147	98
Other debt securities
Fair value
Less than 12 months	$ 19,121	$ 14,576
12 months or longer	0	0
Total	19,121	14,576
Unrealized losses
Less than 12 months	1,406	136
12 months or longer	0	0
Total	$ 1,406	$ 136
No. of holdings | holding	41	24
U.S. Treasury
Fair value
Less than 12 months	$ 8,560	$ 388
12 months or longer	0	0
Total	8,560	388
Unrealized losses
Less than 12 months	150	7
12 months or longer	0	0
Total	$ 150	$ 7
No. of holdings | holding	3	1
Available-for-sale securities
Fair value
Less than 12 months	$ 808,853	$ 375,976
12 months or longer	60,321	23,222
Total	869,174	399,198
Unrealized losses
Less than 12 months	52,531	4,206
12 months or longer	5,713	849
Total	$ 58,244	$ 5,055
No. of holdings | holding	1,466	729