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Summary of Significant Accounting Policies - Various Policies (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Foreign Currency Transactions      
Gain (loss) on revaluation of foreign currency transactions and monetary assets and liabilities $ 21 $ (13) $ 1
Cash and Cash Equivalents      
Outstanding checks included in accounts payable that were reclassified from cash and cash equivalents 60 94  
Accounts Receivable      
Advance cash payments received from third parties to mitigate credit risk 88 180  
Standby letters of credit $ 36 $ 198  
Substantially all trade accounts receivable, net, maximum age of balances past their scheduled invoice date 30 days 30 days  
Allowance for doubtful accounts receivable $ 4 $ 4  
Asset Retirement Obligations      
Fair value of asset retirement obligations $ 35 $ 36