Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	$ 2,219,502		$ 2,061,467
Allowance for Credit Losses	(204)	$ (375)	(397)	$ (985)	$ (878)	$ 0
Gross Unrealized Gains	107,718		138,110
Gross Unrealized Losses	(7,772)		(2,554)
Fair Value	2,319,244		2,196,626
U.S. government
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	154,539		170,110
Gross Unrealized Gains	9,498		13,504
Gross Unrealized Losses	(773)		(257)
Fair Value	163,264		183,357
U.S. Agency
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	28,863		28,902
Gross Unrealized Gains	3,068		3,970
Fair Value	31,931		32,872
Non-U.S. government & agency
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	8,296		10,298
Gross Unrealized Gains	455		667
Gross Unrealized Losses	(122)
Fair Value	8,629		10,965
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	354,748		384,015
Gross Unrealized Gains	13,132		18,789
Gross Unrealized Losses	(2,443)		(733)
Fair Value	365,437		402,071
ABS/CMBS/MBS
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	218,247		213,223
Allowance for Credit Losses			(17)
Gross Unrealized Gains	3,837		5,580
Gross Unrealized Losses	(584)		(413)
Fair Value	221,500		218,373
Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	880,268		753,404
Allowance for Credit Losses	(204)		(380)
Gross Unrealized Gains	50,315		64,501
Gross Unrealized Losses	(2,245)		(933)
Fair Value	928,134		816,592
Municipal
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	574,541		501,515
Gross Unrealized Gains	27,413		31,099
Gross Unrealized Losses	(1,605)		(218)
Fair Value	$ 600,349		$ 532,396