Allowance for Expected Credit Losses - Premiums receivable (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022
Credit Loss [Abstract]
Balance at beginning of period		$ 3,625	$ 2,633
Balance at end of period		4,644	3,625
Allowance for Expected Credit Losses
Balance at beginning of period		35	40
Change for provision of expected credit losses	[1]	(1)	(5)
Balance at end of period		34	35
Write-offs charged against the allowance		$ 3	$ 11

[1] Amounts deemed uncollectible are written-off in operating expenses. For the 2023 and 2022 periods, amounts written off totaled $3 million and $11 million, respectively.