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Debt - Components of Debt (Detail) - USD ($)
$ in Millions
Sep. 30, 2016
Dec. 31, 2015
Debt Instrument [Line Items]    
Debt and capital lease obligations $ 9,261 $ 8,929
Current portion of long-term debt 432 253
Long-term debt, less current portion 8,829 8,676
Revolving Credit Facility [Member]    
Debt Instrument [Line Items]    
Debt and capital lease obligations 295 20
Canadian Credit Facility and Term Loan [Member]    
Debt Instrument [Line Items]    
Debt and capital lease obligations 278 84
Senior Notes [Member]    
Debt Instrument [Line Items]    
Debt and capital lease obligations 6,034 6,050
Tax-exempt Bonds [Member]    
Debt Instrument [Line Items]    
Debt and capital lease obligations 2,346 2,447
Capital Leases and Other [Member]    
Debt Instrument [Line Items]    
Debt and capital lease obligations $ 308 $ 328