Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	$ 3,054,391	$ 2,945,273
Allowance for Credit Losses	(306)	(339)	$ (441)
Gross Unrealized Gains	16,160	2,238
Gross Unrealized Losses	(214,396)	(280,222)
Fair Value	2,855,849	2,666,950
U.S. government
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	312,632	462,884
Gross Unrealized Gains	1,257	8
Gross Unrealized Losses	(5,858)	(8,871)
Fair Value	308,031	454,021
U.S. Agency
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	60,763	75,074
Gross Unrealized Gains	652	26
Gross Unrealized Losses	(1,589)	(2,037)
Fair Value	59,826	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	(918)	(951)
Fair Value	3,882	5,847
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	460,551	373,687
Gross Unrealized Gains	2,636	336
Gross Unrealized Losses	(37,902)	(42,217)
Fair Value	425,285	331,806
ABS/CMBS/MBS
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	308,458	276,126
Allowance for Credit Losses	(3)	(8)
Gross Unrealized Gains	611	62
Gross Unrealized Losses	(27,884)	(35,444)
Fair Value	281,182	240,736
Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	1,273,187	1,122,097
Allowance for Credit Losses	(303)	(331)
Gross Unrealized Gains	8,766	541
Gross Unrealized Losses	(56,631)	(87,977)
Fair Value	1,225,019	1,034,330
Municipal
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	634,000	628,607
Gross Unrealized Gains	2,238	1,265
Gross Unrealized Losses	(83,614)	(102,725)
Fair Value	$ 552,624	$ 527,147