Business acquisitions and development projects - Acquisition of Empire (Details) - CAD CAD in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Goodwill (note 6)	CAD 1,196,234	CAD 306,641	CAD 110,493
Business Acquisition, Pro Forma Information [Abstract]
Revenues	1,977,809	1,908,340
Net earnings attributable to common shareholders	229,976	CAD 213,983
Empire
Business Acquisition [Line Items]
Working capital	55,441
Property, plant and equipment	2,764,441
Goodwill (note 6)	1,010,273
Regulatory assets	318,130
Other assets	58,553
Long-term debt	(1,218,563)
Regulatory liabilities	(195,489)
Pension and OPEB	(105,005)
Deferred income tax liability, net	(562,397)
Other liabilities	(102,759)
Total net assets acquired	2,022,625
Cash and cash equivalent	2,338
Net assets acquired, net of cash and cash equivalent	CAD 2,020,287

Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Working Capital

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Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net of Cash and Cash Equivalents

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The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.

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The pro forma revenue for a period as if the business combination or combinations had been completed at the beginning of the period.

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Amount of currency on hand as well as demand deposits with banks or financial institutions, acquired at the acquisition date. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Amount of deferred tax liability attributable to taxable temporary difference assumed at the acquisition date that are classified as noncurrent.

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Amount of long-term debt due after one year or the normal operating cycle, if longer, assumed at the acquisition date.

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Amount of other liabilities due after one year or the normal operating cycle, if longer, assumed at the acquisition date.

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Amount of other assets expected to be realized or consumed after one year or the normal operating cycle, if longer, acquired at the acquisition date.

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The amount of property, plant, and equipment recognized as of the acquisition date.

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Amount recognized for assets, including goodwill, in excess of (less than) the aggregate liabilities assumed.

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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