Debt - Borrowings and Repayments (Detail) CAD in Millions, $ in Millions	1 Months Ended			3 Months Ended	12 Months Ended
	Nov. 30, 2017 USD ($)	Mar. 31, 2016 CAD	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 CAD	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Aug. 31, 2017 USD ($)
Debt
Net commercial paper borrowings						$ 513
Debt repayments						1,907	$ 2,682	$ 2,764
Loss on early extinguishment of debt						(6)	(4)	(555)
Debt and capital lease principal payments in 2018				$ 737		737
Debt and capital lease principal payments in 2019				302		302
Debt and capital lease principal payments in 2020				754		754
Debt and capital lease principal payments in 2021				549		549
Debt and capital lease principal payments in 2022				592		592
Debt and capital lease principal payments after 2022				6,667		6,667
Commercial Paper Program [Member]
Debt
Net commercial paper borrowings						513
Capital Leases and Other [Member]
Debt
Debt instrument face amount				87		87
Net debt borrowings (repayments)						(68)
Revolving Credit Facility [Member]
Debt
Net line of credit proceeds (repayments)						(703)
Remaining increase in borrowings						277
Canadian Term Loan [Member]
Debt
Debt repayments					CAD 179	137
Senior Notes, Aggregate [Member]
Debt
Loss on early extinguishment of debt								(552)
Early extinguishment of high-coupon senior notes								$ 2,000
Senior Notes, 3.15 Percent, Due November 2027 [Member]
Debt
Debt instrument face amount	$ 750
Net proceeds from issuance of senior notes	$ 745
Interest rate	3.15%
Senior Notes, 6.1 Percent, Due March 2018 [Member]
Debt
Debt repayments	$ 590
Interest rate	6.10%
Loss on early extinguishment of debt				(6)		(6)
Tax Exempt Bonds [Member]
Debt
Debt repayments						127
Debt instrument face amount									$ 175
Debt refunded and reissued				$ 124		$ 124
Minimum [Member] | Senior Notes, Aggregate [Member]
Debt
Interest rate				2.40%		2.40%
Minimum [Member] | Tax Exempt Bonds [Member]
Debt
Interest rate				1.20%		1.20%
Maximum [Member] | Capital Leases and Other [Member]
Debt
Interest rate				12.00%		12.00%
Maximum [Member] | Senior Notes, Aggregate [Member]
Debt
Interest rate				7.75%		7.75%
Maximum [Member] | Tax Exempt Bonds [Member]
Debt
Interest rate				5.70%		5.70%
Canadian Subsidiaries [Member]
Debt
Repayments of intercompany debt | CAD		CAD 370
Cash Flow Hedging [Member] | Foreign Currency Derivatives [Member]
Debt
Cash received derivative termination			$ 67
Derivative termination expense							$ 8