Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Millions	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	$ 27,499		$ 25,616
Gross Unrealized Gains	145		276
Gross Unrealized Losses	(797)		(827)
Allowance for Expected Credit Losses	(27)		(28)
Cost or Amortized Cost	28,178		26,195
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	25,202		23,553
Gross Unrealized Gains	143		275
Gross Unrealized Losses	(796)		(825)
Allowance for Expected Credit Losses	(27)	$ (32)	(28)	$ (61)	$ (56)	$ (41)
Cost or Amortized Cost	25,882		24,131
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	12,043		10,855
Gross Unrealized Gains	84		157
Gross Unrealized Losses	(430)		(464)
Allowance for Expected Credit Losses	(16)	(24)	(20)	(50)	(45)	(30)
Cost or Amortized Cost	12,405		11,182
U.S. government and government agencies
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	5,287		5,814
Gross Unrealized Gains	10		63
Gross Unrealized Losses	(106)		(86)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	5,383		5,837
Asset backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	2,800		2,250
Gross Unrealized Gains	20		11
Gross Unrealized Losses	(35)		(55)
Allowance for Expected Credit Losses	(8)		(5)
Cost or Amortized Cost	2,823		2,299
Non-U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	2,487		2,062
Gross Unrealized Gains	21		33
Gross Unrealized Losses	(110)		(100)
Allowance for Expected Credit Losses	(1)	$ (1)	(1)	$ (3)	$ (3)	$ (2)
Cost or Amortized Cost	2,577		2,130
Commercial mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	1,160		1,213
Gross Unrealized Gains	3		3
Gross Unrealized Losses	(26)		(34)
Allowance for Expected Credit Losses	(2)		(2)
Cost or Amortized Cost	1,185		1,246
Residential mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	1,186		1,103
Gross Unrealized Gains	4		7
Gross Unrealized Losses	(71)		(66)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	1,253		1,162
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	239		256
Gross Unrealized Gains	1		1
Gross Unrealized Losses	(18)		(20)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	256		275
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	2,297		2,063
Gross Unrealized Gains	2		1
Gross Unrealized Losses	(1)		(2)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	$ 2,296		$ 2,064