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Summary of Significant Accounting Policies - Other Balance Sheet Information (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Accounting Policies [Abstract]    
Accounts receivable allowance for doubtful accounts $ 32 $ 32
Property, plant and equipment accumulated depreciation 1,610 1,811
Intangible assets accumulated amortization 1,171 1,149
Out-of-market contracts accumulated amortization $ 0 $ 37