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Summary of Significant Accounting Policies - Various Policies (Details)
$ in Millions
 1 Months Ended 12 Months Ended
Dec. 31, 2017
USD ($)
$ / bbl
Feb. 29, 2016
$ / bbl
Jun. 30, 2014
$ / bbl
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Foreign Currency Transactions            
Gain (loss) on revaluation of foreign currency transactions and monetary assets and liabilities       $ 21 $ (8) $ 21
Cash and Cash Equivalents            
Outstanding checks included in accounts payable that were reclassified from cash and cash equivalents $ 60     60 66  
Accounts Receivable            
NYMEX WTI oil price (in dollars per bbl) | $ / bbl 60.42          
Advance cash payments received from third parties to mitigate credit risk $ 117     117 89  
Standby letters of credit 54     $ 54 $ 66  
Substantially all trade accounts receivable, net, maximum age of balances past their scheduled invoice date       30 days 30 days  
Allowance for doubtful accounts receivable 3     $ 3 $ 3  
Asset Retirement Obligations            
Fair value of asset retirement obligations $ 103     $ 103 $ 44  
Maximum            
Accounts Receivable            
NYMEX WTI oil price (in dollars per bbl) | $ / bbl     107.26      
Minimum            
Accounts Receivable            
NYMEX WTI oil price (in dollars per bbl) | $ / bbl   26.21