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))
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Topic 230
-SubTopic 10
-Section 45
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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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	6 Months Ended
	Jun. 30, 2018 CAD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
Debt
Net commercial paper borrowings		$ 443	$ 253
Debt repayments		196	627
New borrowings		83	$ 86
Revolving Credit Facility [Member]
Debt
Net line of credit proceeds (repayments)		28
Commercial Paper Program [Member]
Debt
Net line of credit proceeds (repayments)		443
Canadian Term Loan [Member]
Debt
Debt repayments	$ 48	37
Tax Exempt Bonds [Member]
Debt
Debt repayments		42
Capital Leases and Other [Member]
Debt
Net debt borrowings (repayments)		$ 67
Minimum [Member] \| Tax Exempt Bonds [Member]
Debt
Interest rate	1.35%	1.35%
Maximum [Member] \| Tax Exempt Bonds [Member]
Debt
Interest rate	5.70%	5.70%
Maximum [Member] \| Capital Leases and Other [Member]
Debt
Interest rate	12.00%	12.00%