Regulatory Assets and Liabilities (Regulated Assets) (Details) - CAD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Regulatory Assets [Line Items]
Regulatory Assets, Current	$ 138	$ 80
Regulatory Assets, Noncurrent	1,238	1,242
Regulatory Assets, Total	1,376	1,322
Deferred income tax regulatory assets [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Total	667	632
Pension and post-retirement medical plan [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Total	345	373
Storm Reserve [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Total	59	0
Environmental remediations [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Total	41	49
Unamortized Defeasance Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Total	32	39
Demand Side Management [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Total	28	32
GBPC Hurricane Matthew restoration [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Total	28	28
Stranded cost recovery [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Total	25	27
Cost Recovery Clauses [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Total	15	15
Deferrals related to derivative instruments [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Total	13	19
Debt Basis Adjustment [Member] [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Total	17	12
Deferred Bond Refinancing Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Total	7	9
Other Regulatory Assets (Liabilities) [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Total	$ 99	$ 87

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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- Definition

The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

+ Details

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- Definition

Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.

+ References

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Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.

+ References

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