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Cash and Equivalents (Details) - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Cash and Cash Equivalents [Line Items]        
Cash and equivalents $ 883 $ 970 $ 1,106 $ 1,053
Operating Cash and Equivalents [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and equivalents 430 320    
Short-term Investments [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and equivalents 400 570    
Cash and Equivalents Held in Joint Ventures [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and equivalents 53 80    
International [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and equivalents 519 761    
International [Member] | Operating Cash and Equivalents [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and equivalents 177 209    
International [Member] | Short-term Investments [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and equivalents 293 481    
International [Member] | Cash and Equivalents Held in Joint Ventures [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and equivalents 49 71    
Domestic [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and equivalents 364 209    
Domestic [Member] | Operating Cash and Equivalents [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and equivalents 253 111    
Domestic [Member] | Short-term Investments [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and equivalents 107 89    
Domestic [Member] | Cash and Equivalents Held in Joint Ventures [Member]        
Cash and Cash Equivalents [Line Items]        
Cash and equivalents $ 4 $ 9