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Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Available For Sale Securities
Cost or Amortized Cost	$ 3,302,684		$ 3,054,391
Allowance for Credit Losses	(234)	$ (228)	(306)	$ (704)	$ (434)	$ (339)
Gross Unrealized Gains	34,419		16,160
Gross Unrealized Losses	(160,796)		(214,396)
Fair Value	3,176,073		2,855,849
U.S. government
Available For Sale Securities
Cost or Amortized Cost	475,022		312,632
Gross Unrealized Gains	8,012		1,257
Gross Unrealized Losses	(2,827)		(5,858)
Fair Value	480,207		308,031
U.S. Agency
Available For Sale Securities
Cost or Amortized Cost	68,861		60,763
Gross Unrealized Gains	1,177		652
Gross Unrealized Losses	(927)		(1,589)
Fair Value	69,111		59,826
Non-U.S. government & agency
Available For Sale Securities
Cost or Amortized Cost	7,958		4,800
Gross Unrealized Gains	62
Gross Unrealized Losses	(759)		(918)
Fair Value	7,261		3,882
Agency MBS
Available For Sale Securities
Cost or Amortized Cost	446,489		460,551
Gross Unrealized Gains	3,877		2,636
Gross Unrealized Losses	(29,976)		(37,902)
Fair Value	420,390		425,285
ABS/CMBS/MBS
Available For Sale Securities
Cost or Amortized Cost	377,578		308,458
Allowance for Credit Losses	(26)		(3)
Gross Unrealized Gains	3,366		611
Gross Unrealized Losses	(19,293)		(27,884)
Fair Value	361,625		281,182
Corporate
Available For Sale Securities
Cost or Amortized Cost	1,372,681		1,273,187
Allowance for Credit Losses	(208)		(303)
Gross Unrealized Gains	15,936		8,766
Gross Unrealized Losses	(35,716)		(56,631)
Fair Value	1,352,693		1,225,019
Municipal
Available For Sale Securities
Cost or Amortized Cost	554,095		634,000
Gross Unrealized Gains	1,989		2,238
Gross Unrealized Losses	(71,298)		(83,614)
Fair Value	$ 484,786		$ 552,624