Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	$ 2,374,256	$ 2,346,267
Allowance for Credit Losses	(594)	(441)	$ (375)	$ (397)
Gross Unrealized Gains	16,702	80,952
Gross Unrealized Losses	(97,986)	(16,891)
Fair Value	2,292,378	2,409,887
U.S. government
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	130,830	127,752
Gross Unrealized Gains	1,314	6,846
Gross Unrealized Losses	(510)	(44)
Fair Value	131,634	134,554
U.S. Agency
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	30,386	30,403
Gross Unrealized Gains	834	2,374
Gross Unrealized Losses	(335)	(17)
Fair Value	30,885	32,760
Non-U.S. government & agency
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	6,797	8,297
Gross Unrealized Gains	135	338
Gross Unrealized Losses	(347)	(154)
Fair Value	6,585	8,481
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	348,318	362,861
Gross Unrealized Gains	1,471	9,277
Gross Unrealized Losses	(15,692)	(4,951)
Fair Value	334,097	367,187
ABS/CMBS/MBS
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	273,349	264,273
Allowance for Credit Losses	(9)
Gross Unrealized Gains	177	2,120
Gross Unrealized Losses	(13,851)	(2,339)
Fair Value	259,666	264,054
Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	953,353	925,394
Allowance for Credit Losses	(585)	(441)
Gross Unrealized Gains	8,925	37,247
Gross Unrealized Losses	(31,357)	(5,105)
Fair Value	930,336	957,095
Municipal
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	631,223	627,287
Gross Unrealized Gains	3,846	22,750
Gross Unrealized Losses	(35,894)	(4,281)
Fair Value	$ 599,175	$ 645,756