Allowance for Expected Credit Losses - Premiums receivable (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Credit Loss [Abstract]
Balance at beginning of period		$ 5,296	$ 3,634	$ 3,625	$ 2,633
Balance at end of period		4,937	3,579	4,937	3,579
Allowance for Expected Credit Losses
Balance at beginning of period		34	38	35	40
Change for provision of expected credit losses	[1]	0	0	(1)	(2)
Balance at end of period		34	38	34	38
Premium receivable, write-offs during the period		$ 0	$ 2	$ 2	$ 7

[1] Amounts deemed uncollectible are written-off in operating expenses. For the 2023 third quarter and 2022 third quarter, amounts written off were nil and $2 million, respectively. For the nine months ended September 30, 2023 and 2022 period, amounts written off were $2 million and $7 million, respectively.