Contractual interest rate for funds borrowed, under the debt agreement.

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(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Topic 230
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Debt - Borrowings and Repayments (Detail) $ in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018 CAD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)
Debt
Net commercial paper borrowings		$ 471	$ 210
Debt repayments		80	541
New borrowings		61	$ 54
Commercial Paper Program [Member]
Debt
Net line of credit proceeds (repayments)		471
Capital Leases and Other [Member]
Debt
Net debt borrowings (repayments)		16
New borrowings		12
Canadian Term Loan [Member]
Debt
Debt repayments	$ 14	11
Tax Exempt Bonds [Member]
Debt
Debt repayments		$ 20
Minimum [Member] \| Senior Notes, Aggregate [Member]
Debt
Interest rate	2.40%	2.40%
Minimum [Member] \| Tax Exempt Bonds [Member]
Debt
Interest rate	1.35%	1.35%
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%