Debt (Detail) CAD in Millions, $ in Millions	1 Months Ended				3 Months Ended	9 Months Ended
	Aug. 31, 2017 USD ($)	Aug. 31, 2016 USD ($)	Mar. 31, 2016 CAD	Mar. 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2017 CAD	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt
Debt maturing within twelve months									$ 1,821
Current portion of long-term debt									850	$ 417
Debt classified as long-term due to intent and ability to refinance on long-term basis									971
Debt and capital lease obligations									9,345	9,310
Net commercial paper borrowings							$ 501
Debt repayments							749	$ 2,210
New borrowings							122	$ 2,536
Canadian Subsidiaries [Member]
Debt
Repayments of intercompany debt | CAD			CAD 370
Foreign Currency Derivatives [Member]
Debt
Cash received in termination of foreign exchange cross currency swaps				$ 67
Expense associated with termination of foreign exchange cross currency swaps					$ 8
Commercial Paper Program [Member]
Debt
Current portion of long-term debt									502
Maximum capacity		$ 1,500
Debt and capital lease obligations									502
Debt term		397 days
Net line of credit repayments							501
Tax Exempt Bonds [Member]
Debt
Current portion of long-term debt									119
Debt with interest rate periods that expiring in the next 12 months									551
Variable-rate tax-exempt bonds									471
Debt and capital lease obligations									2,339	2,304
Debt repayments							40
New borrowings	$ 75
6.1% Senior Notes Mature in March 2018 [Member]
Debt
Current portion of long-term debt									$ 590
Interest rate						6.10%			6.10%
Revolving Credit Facility [Member]
Debt
Maximum capacity									$ 2,250	2,250
Outstanding borrowings under credit facility									0
Letters of credit outstanding									777
Debt and capital lease obligations										426
Unused and available credit capacity									971
Net line of credit repayments							426
Canadian Term Loan and Revolving Credit Facility [Member]
Debt
Debt and capital lease obligations									143	239
Canadian Revolving Credit Facility [Member]
Debt
Maximum capacity | CAD						CAD 50
Outstanding borrowings under credit facility									0
Letters of credit outstanding									0
Canadian Term Loan [Member]
Debt
Maximum term credit | CAD						460
Debt repayments						CAD 142	108
Other Letter of Credit Facilities [Member]
Debt
Letters of credit outstanding									508
Capital Leases and Other [Member]
Debt
Debt and capital lease obligations									328	$ 308
Net debt borrowings (repayments)							53
New borrowings							$ 73
Other Debt [Member]
Debt
Current portion of long-term debt									$ 178