Significant Accounting Policies - Accounts Receivable (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Trade receivables	$ 6,717	$ 6,497
Vendor and manufacturer receivables	7,856	7,315
Premium receivables	2,663	2,259
Other receivables	2,381	1,560
Total accounts receivable, net	19,617	17,631
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	287	162	$ 158
Additions charged to bad debt expense	111	162	139
Write-offs charged to allowance	(79)	(37)	(135)
Ending balance	$ 319	$ 287	$ 162