Investments - Schedule of Realized Gain and Loss and Investments in Unrealized Loss Position (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Investments, Debt and Equity Securities [Abstract]
Realized gross gains	$ 1,219	$ 14,420	$ 4,044
Realized gross losses	8,246	14,864	3,626
Impairment loss	177	12,728	0
Fair Value
Less than 12 months	550,525	2,474,361
12 months or more	3,200,767	1,586,877
Total	3,751,292	4,061,238
Gross Unrealized Losses
Less than 12 months	(5,759)	(198,613)
12 months or more	(330,678)	(248,891)
Total	(336,437)	(447,504)
Realized gross gains	1,219	14,420	4,044
Realized gross losses	8,246	14,864	3,626
Impairment loss	177	12,728	$ 0
U.S. Treasury securities
Fair Value
Less than 12 months	139,398	321,848
12 months or more	355,921	169,795
Total	495,319	491,643
Gross Unrealized Losses
Less than 12 months	(1,075)	(12,381)
12 months or more	(18,053)	(15,695)
Total	(19,128)	(28,076)
U.S. agency securities
Fair Value
Less than 12 months	5,572	7,117
12 months or more	1,623	0
Total	7,195	7,117
Gross Unrealized Losses
Less than 12 months	(2)	(8)
12 months or more	(2)	0
Total	(4)	(8)
U.S. agency mortgage-backed securities
Fair Value
Less than 12 months	129,359	351,310
12 months or more	654,018	415,743
Total	783,377	767,053
Gross Unrealized Losses
Less than 12 months	(1,616)	(34,193)
12 months or more	(100,383)	(80,997)
Total	(101,999)	(115,190)
Municipal debt securities
Fair Value
Less than 12 months	59,301	335,784
12 months or more	297,039	64,766
Total	356,340	400,550
Gross Unrealized Losses
Less than 12 months	(987)	(41,620)
12 months or more	(43,462)	(19,634)
Total	(44,449)	(61,254)
Non-U.S. government securities
Fair Value
Less than 12 months	0	48,071
12 months or more	67,447	14,328
Total	67,447	62,399
Gross Unrealized Losses
Less than 12 months	0	(2,914)
12 months or more	(10,069)	(4,338)
Total	(10,069)	(7,252)
Corporate debt securities
Fair Value
Less than 12 months	119,764	811,217
12 months or more	905,606	421,307
Total	1,025,370	1,232,524
Gross Unrealized Losses
Less than 12 months	(733)	(69,415)
12 months or more	(87,170)	(63,972)
Total	(87,903)	(133,387)
Residential and commercial mortgage securities
Fair Value
Less than 12 months	31,936	265,934
12 months or more	459,789	242,366
Total	491,725	508,300
Gross Unrealized Losses
Less than 12 months	(999)	(22,628)
12 months or more	(52,510)	(43,853)
Total	(53,509)	(66,481)
Asset-backed securities
Fair Value
Less than 12 months	65,195	333,080
12 months or more	459,324	258,572
Total	524,519	591,652
Gross Unrealized Losses
Less than 12 months	(347)	(15,454)
12 months or more	(19,029)	(20,402)
Total	$ (19,376)	$ (35,856)