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 |