Investments - Summary of Realized Gain and Loss and Investments in Unrealized Loss Position (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Realized gross gains	$ 171	$ 18	$ 12,747	$ 768
Realized gross losses	(173)	(271)	(13,264)	(380)
Impairment loss	(469)	$ 0	(7,306)	$ 0
Fair Value
Less than 12 months	3,405,627		3,405,627		$ 2,033,809
12 months or more	575,965		575,965		152,687
Total	3,981,592		3,981,592		2,186,496
Gross Unrealized Losses
Less than 12 months	(259,311)		(259,311)		(28,074)
12 months or more	(76,091)		(76,091)		(5,393)
Total	(335,402)		(335,402)		(33,467)
U.S. Treasury securities
Fair Value
Less than 12 months	351,173		351,173		207,122
12 months or more	75,530		75,530		28,012
Total	426,703		426,703		235,134
Gross Unrealized Losses
Less than 12 months	(14,844)		(14,844)		(2,170)
12 months or more	(4,816)		(4,816)		(796)
Total	(19,660)		(19,660)		(2,966)
U.S. agency mortgage-backed securities
Fair Value
Less than 12 months	580,870		580,870		582,108
12 months or more	195,262		195,262		26,131
Total	776,132		776,132		608,239
Gross Unrealized Losses
Less than 12 months	(55,313)		(55,313)		(9,414)
12 months or more	(28,468)		(28,468)		(699)
Total	(83,781)		(83,781)		(10,113)
Municipal debt securities
Fair Value
Less than 12 months	379,124		379,124		91,719
12 months or more	6,161		6,161		312
Total	385,285		385,285		92,031
Gross Unrealized Losses
Less than 12 months	(41,896)		(41,896)		(1,281)
12 months or more	(896)		(896)		(6)
Total	(42,792)		(42,792)		(1,287)
Non-U.S. government securities:
Fair Value
Less than 12 months	24,663		24,663		22,986
12 months or more	12,551		12,551		0
Total	37,214		37,214		22,986
Gross Unrealized Losses
Less than 12 months	(2,893)		(2,893)		(855)
12 months or more	(3,615)		(3,615)		0
Total	(6,508)		(6,508)		(855)
Corporate debt securities
Fair Value
Less than 12 months	1,087,132		1,087,132		522,120
12 months or more	128,734		128,734		46,875
Total	1,215,866		1,215,866		568,995
Gross Unrealized Losses
Less than 12 months	(86,308)		(86,308)		(7,200)
12 months or more	(23,261)		(23,261)		(2,265)
Total	(109,569)		(109,569)		(9,465)
Residential and commercial mortgage securities
Fair Value
Less than 12 months	444,392		444,392		268,617
12 months or more	93,519		93,519		38,256
Total	537,911		537,911		306,873
Gross Unrealized Losses
Less than 12 months	(32,637)		(32,637)		(5,200)
12 months or more	(12,690)		(12,690)		(1,467)
Total	(45,327)		(45,327)		(6,667)
Asset-backed securities
Fair Value
Less than 12 months	538,273		538,273		339,137
12 months or more	64,208		64,208		13,101
Total	602,481		602,481		352,238
Gross Unrealized Losses
Less than 12 months	(25,420)		(25,420)		(1,954)
12 months or more	(2,345)		(2,345)		(160)
Total	$ (27,765)		$ (27,765)		$ (2,114)