Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Millions	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	$ 31,775		$ 25,616
Gross Unrealized Gains	524		276
Gross Unrealized Losses	(533)		(827)
Allowance for Expected Credit Losses	(19)		(28)
Cost or Amortized Cost	31,803		26,195
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	28,434		23,553
Gross Unrealized Gains	518		275
Gross Unrealized Losses	(531)		(825)
Allowance for Expected Credit Losses	(19)	$ (27)	(28)	$ (57)	$ (61)	$ (41)
Cost or Amortized Cost	28,466		24,131
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	13,996		10,855
Gross Unrealized Gains	339		157
Gross Unrealized Losses	(294)		(464)
Allowance for Expected Credit Losses	(12)	(16)	(20)	(47)	(50)	(30)
Cost or Amortized Cost	13,963		11,182
U.S. government and government agencies
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	5,863		5,814
Gross Unrealized Gains	53		63
Gross Unrealized Losses	(48)		(86)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	5,858		5,837
Asset backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	2,989		2,250
Gross Unrealized Gains	27		11
Gross Unrealized Losses	(27)		(55)
Allowance for Expected Credit Losses	(5)		(5)
Cost or Amortized Cost	2,994		2,299
Non-U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	2,853		2,062
Gross Unrealized Gains	75		33
Gross Unrealized Losses	(84)		(100)
Allowance for Expected Credit Losses	(1)	$ (1)	(1)	$ (2)	$ (3)	$ (2)
Cost or Amortized Cost	2,863		2,130
Commercial mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	1,146		1,213
Gross Unrealized Gains	8		3
Gross Unrealized Losses	(17)		(34)
Allowance for Expected Credit Losses	(1)		(2)
Cost or Amortized Cost	1,156		1,246
Residential mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	1,313		1,103
Gross Unrealized Gains	14		7
Gross Unrealized Losses	(50)		(66)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	1,349		1,162
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	274		256
Gross Unrealized Gains	2		1
Gross Unrealized Losses	(11)		(20)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	283		275
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	3,341		2,063
Gross Unrealized Gains	6		1
Gross Unrealized Losses	(2)		(2)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	$ 3,337		$ 2,064