Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Available For Sale Securities
Cost or Amortized Cost	$ 3,082,273	$ 3,054,391
Allowance for Credit Losses	(237)	(306)	$ (465)	$ (339)
Gross Unrealized Gains	9,155	16,160
Gross Unrealized Losses	(227,870)	(214,396)
Fair Value	2,863,321	2,855,849
U.S. government
Available For Sale Securities
Cost or Amortized Cost	337,014	312,632
Gross Unrealized Gains	388	1,257
Gross Unrealized Losses	(7,843)	(5,858)
Fair Value	329,559	308,031
U.S. Agency
Available For Sale Securities
Cost or Amortized Cost	59,956	60,763
Gross Unrealized Gains	444	652
Gross Unrealized Losses	(1,945)	(1,589)
Fair Value	58,455	59,826
Non-U.S. government & agency
Available For Sale Securities
Cost or Amortized Cost	4,800	4,800
Gross Unrealized Losses	(956)	(918)
Fair Value	3,844	3,882
Agency MBS
Available For Sale Securities
Cost or Amortized Cost	456,358	460,551
Gross Unrealized Gains	1,208	2,636
Gross Unrealized Losses	(43,138)	(37,902)
Fair Value	414,428	425,285
ABS/CMBS/MBS
Available For Sale Securities
Cost or Amortized Cost	315,414	308,458
Allowance for Credit Losses	(2)	(3)
Gross Unrealized Gains	970	611
Gross Unrealized Losses	(26,906)	(27,884)
Fair Value	289,476	281,182
Corporate Debt
Available For Sale Securities
Cost or Amortized Cost	1,277,258	1,273,187
Allowance for Credit Losses	(235)	(303)
Gross Unrealized Gains	4,732	8,766
Gross Unrealized Losses	(60,708)	(56,631)
Fair Value	1,221,047	1,225,019
Municipal
Available For Sale Securities
Cost or Amortized Cost	631,473	634,000
Gross Unrealized Gains	1,413	2,238
Gross Unrealized Losses	(86,374)	(83,614)
Fair Value	$ 546,512	$ 552,624