Allowance for Expected Credit Losses - Premiums receivable (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Credit Loss [Abstract]
Balance at beginning of period		$ 5,765	$ 4,513	$ 4,644	$ 3,625
Balance at end of period		6,268	5,296	6,268	5,296
Allowance for Expected Credit Losses
Balance at beginning of period		32	36	34	35
Change for provision of expected credit losses	[1]	4	(2)	2	(1)
Balance at end of period		36	34	36	34
Premium receivable, write-offs during the period		$ 0	$ 1	$ 0	$ 2

[1] Amounts deemed uncollectible are written-off in operating expenses. For the 2024 second quarter and 2023 second quarter, amounts written off were nil and $1 million, respectively. For the six months ended June 30, 2024 and 2023 period, amounts written off were nil and $2 million, respectively.