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Accounting Pronouncements (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Operating lease assets (Note 16) $ 1,610 $ 1,812
Operating lease liabilities (Note 16) $ 1,283 1,471
Minimum [Member]    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Finance lease, renewal term 12 months  
Operating lease, renewal term 12 months  
ASU 2016-02 Adopted [Member]    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Operating lease assets (Note 16)   2,000
Operating lease liabilities (Note 16)   $ 2,000
ASU 2016-02 Adopted [Member] | Minimum [Member]    
New Accounting Pronouncements or Change in Accounting Principle [Line Items]    
Finance lease, renewal term   12 months
Operating lease, renewal term   12 months