Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	$ 2,310,314		$ 2,061,467
Allowance for Credit Losses	(226)	$ (204)	(397)	$ (632)	$ (985)	$ 0
Gross Unrealized Gains	96,654		138,110
Gross Unrealized Losses	(12,073)		(2,554)
Fair Value	2,394,669		2,196,626
U.S. government
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	139,441		170,110
Gross Unrealized Gains	8,549		13,504
Gross Unrealized Losses	(797)		(257)
Fair Value	147,193		183,357
U.S. Agency
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	28,844		28,902
Gross Unrealized Gains	2,781		3,970
Fair Value	31,625		32,872
Non-U.S. government & agency
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	8,297		10,298
Gross Unrealized Gains	407		667
Gross Unrealized Losses	(183)
Fair Value	8,521		10,965
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	385,400		384,015
Gross Unrealized Gains	12,179		18,789
Gross Unrealized Losses	(3,309)		(733)
Fair Value	394,270		402,071
ABS/CMBS/MBS
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	247,598		213,223
Allowance for Credit Losses			(17)
Gross Unrealized Gains	3,915		5,580
Gross Unrealized Losses	(955)		(413)
Fair Value	250,558		218,373
Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	896,178		753,404
Allowance for Credit Losses	(226)		(380)
Gross Unrealized Gains	45,230		64,501
Gross Unrealized Losses	(2,912)		(933)
Fair Value	938,270		816,592
Municipal
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	604,556		501,515
Gross Unrealized Gains	23,593		31,099
Gross Unrealized Losses	(3,917)		(218)
Fair Value	$ 624,232		$ 532,396