Investments - Amortized Cost and Fair Value of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Available For Sale Securities
Cost or Amortized Cost	$ 3,217,866		$ 3,054,391
Allowance for Credit Losses	(228)	$ (237)	(306)	$ (434)	$ (465)	$ (339)
Gross Unrealized Gains	7,555		16,160
Gross Unrealized Losses	(235,666)		(214,396)
Fair Value	2,989,527		2,855,849
U.S. government
Available For Sale Securities
Cost or Amortized Cost	442,734		312,632
Gross Unrealized Gains	258		1,257
Gross Unrealized Losses	(8,192)		(5,858)
Fair Value	434,800		308,031
U.S. Agency
Available For Sale Securities
Cost or Amortized Cost	60,058		60,763
Gross Unrealized Gains	326		652
Gross Unrealized Losses	(2,075)		(1,589)
Fair Value	58,309		59,826
Non-U.S. government & agency
Available For Sale Securities
Cost or Amortized Cost	4,800		4,800
Gross Unrealized Losses	(972)		(918)
Fair Value	3,828		3,882
Agency MBS
Available For Sale Securities
Cost or Amortized Cost	458,944		460,551
Gross Unrealized Gains	915		2,636
Gross Unrealized Losses	(45,184)		(37,902)
Fair Value	414,675		425,285
ABS/CMBS/MBS
Available For Sale Securities
Cost or Amortized Cost	345,072		308,458
Allowance for Credit Losses	(2)		(3)
Gross Unrealized Gains	1,367		611
Gross Unrealized Losses	(26,376)		(27,884)
Fair Value	320,061		281,182
Corporate Debt
Available For Sale Securities
Cost or Amortized Cost	1,310,888		1,273,187
Allowance for Credit Losses	(226)		(303)
Gross Unrealized Gains	3,620		8,766
Gross Unrealized Losses	(63,502)		(56,631)
Fair Value	1,250,780		1,225,019
Municipal
Available For Sale Securities
Cost or Amortized Cost	595,370		634,000
Gross Unrealized Gains	1,069		2,238
Gross Unrealized Losses	(89,365)		(83,614)
Fair Value	$ 507,074		$ 552,624