Allowance for Expected Credit Losses (Details) - Changes in the Allowance for Expected Credit Losses on Premiums Receivable - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Credit Loss [Abstract]
Premiums receivable (net of allowance for expected credit losses), balance, beginning of period	$ 8,085	$ 7,829
Premiums receivable (net of allowance for expected credit losses), balance, end of period	8,593	8,167
Premiums receivable, allowance for expected credit losses [Roll Forward]
Premiums receivable, allowance for expected credit losses, beginning balance	107	105
Current period change for expected credit losses	18	16
Write-offs of uncollectible premiums receivable	36	11
Premiums receivable, allowance for expected credit losses, ending balance	$ 89	$ 110