Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Millions	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	$ 34,259		$ 29,819
Gross Unrealized Gains	447		181
Gross Unrealized Losses	(338)		(694)
Allowance for Expected Credit Losses	(21)		(22)
Cost or Amortized Cost	34,171		30,354
Fixed maturities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	31,908		27,035
Gross Unrealized Gains	445		179
Gross Unrealized Losses	(337)		(692)
Allowance for Expected Credit Losses	(21)	$ (28)	(22)	$ (19)	$ (27)	$ (28)
Cost or Amortized Cost	31,821		27,570
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	15,005		12,487
Gross Unrealized Gains	301		110
Gross Unrealized Losses	(173)		(346)
Allowance for Expected Credit Losses	(14)	(16)	(12)	(12)	(16)	(20)
Cost or Amortized Cost	14,891		12,735
U.S. government and government agencies
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	6,324		6,710
Gross Unrealized Gains	23		8
Gross Unrealized Losses	(31)		(149)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	6,332		6,851
Asset backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	3,149		2,900
Gross Unrealized Gains	22		19
Gross Unrealized Losses	(19)		(32)
Allowance for Expected Credit Losses	(6)		(8)
Cost or Amortized Cost	3,152		2,921
Non-U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	3,241		2,538
Gross Unrealized Gains	59		30
Gross Unrealized Losses	(81)		(107)
Allowance for Expected Credit Losses	(1)	$ (2)	(1)	$ (1)	$ (1)	$ (1)
Cost or Amortized Cost	3,264		2,616
Commercial mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	1,249		1,058
Gross Unrealized Gains	10		6
Gross Unrealized Losses	(6)		(11)
Allowance for Expected Credit Losses	0		(1)
Cost or Amortized Cost	1,245		1,064
Residential mortgage backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	2,766		1,079
Gross Unrealized Gains	30		6
Gross Unrealized Losses	(22)		(31)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	2,758		1,104
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	174		263
Gross Unrealized Gains	0		0
Gross Unrealized Losses	(5)		(16)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	179		279
Short-term investments
Debt Securities, Available-for-sale [Line Items]
Estimated Fair Value	2,351		2,784
Gross Unrealized Gains	2		2
Gross Unrealized Losses	(1)		(2)
Allowance for Expected Credit Losses	0		0
Cost or Amortized Cost	$ 2,350		$ 2,784