Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	$ 2,753,631		$ 2,346,267
Allowance for Credit Losses	(327)	$ (292)	(441)	$ (226)	$ (204)	$ (397)
Gross Unrealized Gains	443		80,952
Gross Unrealized Losses	(301,708)		(16,891)
Fair Value	2,452,039		2,409,887
U.S. government
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	349,419		127,752
Gross Unrealized Gains			6,846
Gross Unrealized Losses	(8,977)		(44)
Fair Value	340,442		134,554
U.S. Agency
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	59,048		30,403
Gross Unrealized Gains	9		2,374
Gross Unrealized Losses	(2,122)		(17)
Fair Value	56,935		32,760
Non-U.S. government & agency
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	6,798		8,297
Gross Unrealized Gains			338
Gross Unrealized Losses	(1,086)		(154)
Fair Value	5,712		8,481
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	360,429		362,861
Gross Unrealized Gains	172		9,277
Gross Unrealized Losses	(43,572)		(4,951)
Fair Value	317,029		367,187
ABS/CMBS/MBS
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	277,401		264,273
Allowance for Credit Losses	(9)
Gross Unrealized Gains	21		2,120
Gross Unrealized Losses	(35,901)		(2,339)
Fair Value	241,512		264,054
Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	1,067,473		925,394
Allowance for Credit Losses	(318)		(441)
Gross Unrealized Gains	47		37,247
Gross Unrealized Losses	(103,333)		(5,105)
Fair Value	963,869		957,095
Municipal
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	633,063		627,287
Gross Unrealized Gains	194		22,750
Gross Unrealized Losses	(106,717)		(4,281)
Fair Value	$ 526,540		$ 645,756