Allowance for Expected Credit Losses (Details) - Changes in the Allowance for Expected Credit Losses on Premiums Receivable - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Credit Loss [Abstract]
Premiums receivable (net of allowance for expected credit losses), balance, beginning of period	$ 10,327	$ 9,132	$ 8,922	$ 8,085
Premiums receivable (net of allowance for expected credit losses), balance, end of period	10,345	8,886	10,345	8,886
Premiums receivable, allowance for expected credit losses [Roll Forward]
Premiums receivable, allowance for expected credit losses, beginning balance	72	89	77	107
Current period change for expected credit losses	11	14	28	49
Write-offs of uncollectible premiums receivable	15	17	37	70
Premiums receivable, allowance for expected credit losses, ending balance	$ 68	$ 86	$ 68	$ 86