Long-term Debt (Commercial Paper) (Details) CAD in Millions, $ in Millions	12 Months Ended
Dec. 31, 2017 CAD	Dec. 31, 2016 CAD	Dec. 31, 2015 CAD	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Commercial Paper
Commercial paper, maximum borrowing capacity	CAD 1,300
Commercial paper, outstanding	CAD 955	CAD 605		$ 760	$ 451
Commercial paper, weighted average interest rate	1.36%	0.65%		1.36%	0.65%
Commercial paper, maturity	90 days
Issuances and repayments of commercial paper
Issuances of commerical paper	CAD 4,539	CAD 3,656	CAD 2,624
Repayments of commercial paper	(4,160)	(3,519)	(2,173)
Net issuance (repayment) of commercial paper	CAD 379	CAD 137	CAD 451

No definition available.

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- Definition

Maximum borrowing capacity that the Company is enabling to issue under the commercial paper program.

+ References

No definition available.

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- Definition

Period of time between issuance and maturity of commercial paper, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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- Definition

Weighted average interest rate of commercial paper outstanding.

+ References

No definition available.

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- Definition

Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

+ 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(19)(a)(3))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13)(2))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- Definition

The cash inflow from borrowing by issuing commercial paper.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Financing Activities
-URI http://asc.fasb.org/extlink&oid=6513228

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
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-Section 45
-Paragraph 14
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-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3255-108585

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- Definition

The net cash inflow or cash outflow from issuing (borrowing) and repaying commercial paper.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
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-Paragraph 9
-Subparagraph c
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- References

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The cash outflow due to repaying amounts borrowed by issuing commercial paper.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Financing Activities
-URI http://asc.fasb.org/extlink&oid=6513228

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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