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Lease Commitments - Lease Cost (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Leases [Abstract]      
Operating lease costs $ 2,621    
Finance lease costs 66    
Total lease costs $ 2,687    
Net rental expense   $ 816 $ 721