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Summary of Significant Accounting Policies - Minimum Volume Commitments (Details) - Minimum Volume Commitments - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Minimum Volume Commitments    
Total counterparty deficiencies $ 57 $ 66
Deferred revenue 37 54
Deficiencies not yet billed or collected $ 20 $ 12