Debt - Components of Debt (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Debt and capital lease obligations	$ 9,530	$ 8,929
Current portion of long-term debt	614	253
Long-term debt, less current portion	8,916	8,676
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Debt and capital lease obligations		20
Letter of Credit Facilities [Member]
Debt Instrument [Line Items]
Debt and capital lease obligations	0	0
Canadian Credit Facility and Term Loan [Member]
Debt Instrument [Line Items]
Debt and capital lease obligations	313	84
Senior Notes [Member]
Debt Instrument [Line Items]
Debt and capital lease obligations	6,534	6,050
Tax-exempt Bonds [Member]
Debt Instrument [Line Items]
Debt and capital lease obligations	2,350	2,447
Capital Leases and Other [Member]
Debt Instrument [Line Items]
Debt and capital lease obligations	$ 333	$ 328

Sum of the carrying values as of the balance sheet date of all debt, including all short-term borrowings, long-term debt, and capital lease obligations.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.13,16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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Amount of long-term debt and capital lease obligation due after one year or beyond the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Staff Accounting Bulletin (SAB)
-Number Topic 6
-Section H

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