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Significant Accounting Policies - Accounts Receivable (Details) - USD ($)
$ in Millions
Jun. 30, 2021
Dec. 31, 2020
Accounting Policies [Abstract]    
Trade receivables $ 8,021 $ 7,101
Vendor and manufacturer receivables 10,955 9,815
Premium receivables 2,942 2,628
Other receivables 2,246 2,198
Total accounts receivable, net 24,164 21,742
Allowance for credit losses $ 367 $ 358