Allowance for Expected Credit Losses (Details) - Changes in the Allowance for Expected Credit Losses on Premiums Receivable $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($)
Credit Loss [Abstract]
Premiums receivable (net of allowance for expected credit losses), balance, beginning of year	$ 7,909
Premiums receivable (net of allowance for expected credit losses), balance, end of period	8,202
Premiums receivable, allowance for expected credit losses [Roll Forward]
Premiums receivable, allowance for expected credit losses, beginning balance	49
Current period change for expected credit losses	43
Write-offs of uncollectible premiums receivable	14
Premiums receivable, allowance for expected credit losses, ending balance	$ 78