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Summary of Significant Accounting Policies - Recent Accounting Developments (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Cost of operations $ 2,051 $ 1,841 $ 3,609 $ 3,704
Other income, net $ (20) 14 $ (23) 26
ASU 2017-07        
New Accounting Pronouncements or Change in Accounting Principle [Line Items]        
Cost of operations   4   8
Other income, net   $ 4   $ 8