Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	$ 2,945,273	$ 2,346,267
Allowance for Credit Losses	(339)	(441)	$ (397)
Gross Unrealized Gains	2,238	80,952
Gross Unrealized Losses	(280,222)	(16,891)
Fair Value	2,666,950	2,409,887
U.S. government
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	462,884	127,752
Gross Unrealized Gains	8	6,846
Gross Unrealized Losses	(8,871)	(44)
Fair Value	454,021	134,554
U.S. Agency
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	75,074	30,403
Gross Unrealized Gains	26	2,374
Gross Unrealized Losses	(2,037)	(17)
Fair Value	73,063	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	(951)	(154)
Fair Value	5,847	8,481
Agency MBS
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	373,687	362,861
Gross Unrealized Gains	336	9,277
Gross Unrealized Losses	(42,217)	(4,951)
Fair Value	331,806	367,187
ABS/CMBS/MBS
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	276,126	264,273
Allowance for Credit Losses	(8)
Gross Unrealized Gains	62	2,120
Gross Unrealized Losses	(35,444)	(2,339)
Fair Value	240,736	264,054
Corporate Debt
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	1,122,097	925,394
Allowance for Credit Losses	(331)	(441)
Gross Unrealized Gains	541	37,247
Gross Unrealized Losses	(87,977)	(5,105)
Fair Value	1,034,330	957,095
Municipal
Schedule Of Available For Sale Securities [Line Items]
Cost or Amortized Cost	628,607	627,287
Gross Unrealized Gains	1,265	22,750
Gross Unrealized Losses	(102,725)	(4,281)
Fair Value	$ 527,147	$ 645,756

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
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