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Summary of Significant Accounting Policies - Recent Accounting Developments (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Jan. 01, 2019
Dec. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Operating lease liabilities $ 603    
Operating lease right-of-use assets $ 517   $ 0
ASU 2016-02      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Operating lease liabilities   $ 404  
Operating lease right-of-use assets   $ 321