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Lease Commitments - Lease Cost (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Leases [Abstract]      
Operating lease costs $ 2,551 $ 2,621  
Finance lease costs 45 66  
Total lease costs $ 2,596 $ 2,687  
Net rental expense     $ 816