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Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Mar. 31, 2022	Dec. 31, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	$ 2,480,480		$ 2,346,267
Allowance for Credit Losses	(292)	$ (594)	(441)	$ (204)	$ (375)	$ (397)
Gross Unrealized Gains	3,371		80,952
Gross Unrealized Losses	(202,650)		(16,891)
Fair Value	2,280,909		2,409,887
U.S. government
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	183,368		127,752
Gross Unrealized Gains	220		6,846
Gross Unrealized Losses	(1,910)		(44)
Fair Value	181,678		134,554
U.S. Agency
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	34,220		30,403
Gross Unrealized Gains	190		2,374
Gross Unrealized Losses	(837)		(17)
Fair Value	33,573		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			338
Gross Unrealized Losses	(729)		(154)
Fair Value	6,068		8,481
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	338,639		362,861
Gross Unrealized Gains	591		9,277
Gross Unrealized Losses	(26,509)		(4,951)
Fair Value	312,721		367,187
ABS/CMBS/MBS
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	271,628		264,273
Allowance for Credit Losses	(10)
Gross Unrealized Gains	82		2,120
Gross Unrealized Losses	(25,355)		(2,339)
Fair Value	246,345		264,054
Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	1,011,726		925,394
Allowance for Credit Losses	(282)		(441)
Gross Unrealized Gains	428		37,247
Gross Unrealized Losses	(70,143)		(5,105)
Fair Value	941,729		957,095
Municipal
Schedule Of Available For Sale Securities [Line Items]
Total available-for-sale	634,102		627,287
Gross Unrealized Gains	1,860		22,750
Gross Unrealized Losses	(77,167)		(4,281)
Fair Value	$ 558,795		$ 645,756