Investments - Schedule of Gross Unrealized Losses and Fair Value by Type and Length of Time Held in a Continuous Unrealized Loss Position (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2024 USD ($) contract	Dec. 31, 2023 USD ($) contract
Total unrealized losses
Fixed income securities, at fair value	$ 50,777,000,000	$ 48,865,000,000
Fixed income securities
Less than 12 months
Number of  issues | contract	1,096	611
Fair value	$ 10,766,000,000	$ 5,274,000,000
Unrealized losses	$ (183,000,000)	$ (91,000,000)
12 months or more
Number of  issues | contract	3,858	4,232
Fair value	$ 19,903,000,000	$ 23,017,000,000
Unrealized losses	(1,285,000,000)	(1,343,000,000)
Total unrealized losses
Total unrealized losses	(1,468,000,000)	(1,434,000,000)
Fixed income securities, at fair value	3,000,000	32,000,000
Fixed income securities, unrealized losses	0	3,000,000
Fixed income securities, credit loss allowance	$ 1,000,000	$ 8,000,000
U.S. government and agencies
Less than 12 months
Number of  issues | contract	118	63
Fair value	$ 3,867,000,000	$ 2,554,000,000
Unrealized losses	$ (85,000,000)	$ (38,000,000)
12 months or more
Number of  issues | contract	104	117
Fair value	$ 2,196,000,000	$ 2,513,000,000
Unrealized losses	(83,000,000)	(81,000,000)
Total unrealized losses
Total unrealized losses	(168,000,000)	(119,000,000)
Fixed income securities, at fair value	$ 10,030,000,000	$ 8,619,000,000
Municipal
Less than 12 months
Number of  issues | contract	397	271
Fair value	$ 915,000,000	$ 400,000,000
Unrealized losses	$ (9,000,000)	$ (4,000,000)
12 months or more
Number of  issues | contract	1,600	1,784
Fair value	$ 2,106,000,000	$ 2,245,000,000
Unrealized losses	(151,000,000)	(148,000,000)
Total unrealized losses
Total unrealized losses	(160,000,000)	(152,000,000)
Fixed income securities, at fair value	$ 5,392,000,000	$ 6,006,000,000
Corporate
Less than 12 months
Number of  issues | contract	493	251
Fair value	$ 5,317,000,000	$ 2,225,000,000
Unrealized losses	$ (84,000,000)	$ (48,000,000)
12 months or more
Number of  issues | contract	1,962	2,106
Fair value	$ 15,133,000,000	$ 17,319,000,000
Unrealized losses	(1,030,000,000)	(1,095,000,000)
Total unrealized losses
Total unrealized losses	(1,114,000,000)	(1,143,000,000)
Fixed income securities, at fair value	$ 32,269,000,000	$ 31,205,000,000
Foreign government
Less than 12 months
Number of  issues | contract	40	7
Fair value	$ 453,000,000	$ 31,000,000
Unrealized losses	$ (3,000,000)	$ 0
12 months or more
Number of  issues | contract	74	75
Fair value	$ 327,000,000	$ 356,000,000
Unrealized losses	(16,000,000)	(13,000,000)
Total unrealized losses
Total unrealized losses	(19,000,000)	(13,000,000)
Fixed income securities, at fair value	$ 1,240,000,000	$ 1,290,000,000
ABS
Less than 12 months
Number of  issues | contract	48	19
Fair value	$ 214,000,000	$ 64,000,000
Unrealized losses	$ (2,000,000)	$ (1,000,000)
12 months or more
Number of  issues | contract	118	150
Fair value	$ 141,000,000	$ 584,000,000
Unrealized losses	(5,000,000)	(6,000,000)
Total unrealized losses
Total unrealized losses	(7,000,000)	(7,000,000)
Fixed income securities, at fair value	$ 1,846,000,000	$ 1,745,000,000
Investment grade fixed income securities
Less than 12 months
Number of  issues | contract	1,032	568
Fair value	$ 10,551,000,000	$ 5,061,000,000
Unrealized losses	$ (173,000,000)	$ (83,000,000)
12 months or more
Number of  issues | contract	3,518	3,864
Fair value	$ 17,647,000,000	$ 20,429,000,000
Unrealized losses	(1,121,000,000)	(1,151,000,000)
Total unrealized losses
Total unrealized losses	$ (1,294,000,000)	$ (1,234,000,000)
Below investment grade fixed income securities
Less than 12 months
Number of  issues | contract	64	43
Fair value	$ 215,000,000	$ 213,000,000
Unrealized losses	$ (10,000,000)	$ (8,000,000)
12 months or more
Number of  issues | contract	340	368
Fair value	$ 2,256,000,000	$ 2,588,000,000
Unrealized losses	(164,000,000)	(192,000,000)
Total unrealized losses
Total unrealized losses	$ (174,000,000)	$ (200,000,000)