Allowance for Expected Credit Losses (Details) - Changes in the Allowance for Expected Credit Losses on Premiums Receivable - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Credit Loss [Abstract]
Premiums receivable (net of allowance for expected credit losses), balance, beginning of period	$ 8,167	$ 8,202	$ 7,829	$ 7,909
Premiums receivable (net of allowance for expected credit losses), balance, end of period	8,555	8,459	8,555	8,459
Premiums receivable, allowance for expected credit losses [Roll Forward]
Premiums receivable, allowance for expected credit losses, beginning balance	110	78	105	49
Current period change for expected credit losses	14	24	30	67
Write-offs of uncollectible premiums receivable	19	8	30	22
Premiums receivable, allowance for expected credit losses, ending balance	$ 105	$ 94	$ 105	$ 94