Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	$ 3,046,785		$ 2,945,273
Allowance for Credit Losses	(704)	$ (434)	(339)	$ (327)	$ (292)	$ (441)
Gross Unrealized Gains	1,109		2,238
Gross Unrealized Losses	(329,563)		(280,222)
Fair Value	2,717,627		2,666,950
U.S. government
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	334,650		462,884
Gross Unrealized Gains			8
Gross Unrealized Losses	(13,039)		(8,871)
Fair Value	321,611		454,021
U.S. Agency
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	51,477		75,074
Gross Unrealized Gains			26
Gross Unrealized Losses	(3,109)		(2,037)
Fair Value	48,368		73,063
Non-U.S. government & agency
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	4,800		6,798
Gross Unrealized Losses	(989)		(951)
Fair Value	3,811		5,847
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	457,496		373,687
Gross Unrealized Gains	7		336
Gross Unrealized Losses	(58,122)		(42,217)
Fair Value	399,381		331,806
ABS/CMBS/MBS
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	302,659		276,126
Allowance for Credit Losses	(4)		(8)
Gross Unrealized Gains	65		62
Gross Unrealized Losses	(36,941)		(35,444)
Fair Value	265,779		240,736
Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	1,253,152		1,122,097
Allowance for Credit Losses	(700)		(331)
Gross Unrealized Gains	952		541
Gross Unrealized Losses	(100,020)		(87,977)
Fair Value	1,153,384		1,034,330
Municipal
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	642,551		628,607
Gross Unrealized Gains	85		1,265
Gross Unrealized Losses	(117,343)		(102,725)
Fair Value	$ 525,293		$ 527,147