Allowance for Expected Credit Losses (Details) - Changes in the Allowance for Expected Credit Losses on Premiums Receivable - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Credit Loss [Abstract]
Premiums receivable (net of allowance for expected credit losses), balance, beginning of period	$ 7,829	$ 7,909
Premiums receivable (net of allowance for expected credit losses), balance, end of period	8,167	7,829
Premiums receivable, allowance for expected credit losses [Roll Forward]
Premiums receivable, allowance for expected credit losses, beginning balance	105	49
Current period change for expected credit losses	16	103
Write-offs of uncollectible premiums receivable	11	47
Premiums receivable, allowance for expected credit losses, ending balance	$ 110	$ 105