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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Credit Loss [Abstract]
Premiums receivable (net of allowance for expected credit losses), balance, beginning of period	$ 8,555	$ 8,459	$ 7,829	$ 7,909
Premiums receivable (net of allowance for expected credit losses), balance, end of period	8,289	8,225	8,289	8,225
Premiums receivable, allowance for expected credit losses [Roll Forward]
Premiums receivable, allowance for expected credit losses, beginning balance	105	94	105	49
Current period change for expected credit losses	15	17	45	84
Write-offs of uncollectible premiums receivable	18	15	48	37
Premiums receivable, allowance for expected credit losses, ending balance	$ 102	$ 96	$ 102	$ 96