Significant Accounting Policies - Accounts Receivable (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Accounting Policies [Abstract]
Trade receivables	$ 7,748	$ 7,101
Vendor and manufacturer receivables	10,556	9,815
Premium receivables	3,101	2,628
Other receivables	2,450	2,198
Total accounts receivable, net	23,855	21,742
Allowance for credit losses	$ 375	$ 358