Income taxes - Income Tax Expense (Recovery) Attributable to Income (Loss) (Detail) - CAD CAD in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Income Tax Expenses [Line Items]
Income tax expenses, current	CAD 9,908	CAD 8,461
Income tax expenses, deferred	85,286	28,675
Income tax expense	95,194	37,136
Canada
Income Tax Expenses [Line Items]
Income tax expenses, current	4,277	7,533
Income tax expenses, deferred	(18,390)	(10,501)
Income tax expense	(14,113)	(2,968)
United States
Income Tax Expenses [Line Items]
Income tax expenses, current	5,631	928
Income tax expenses, deferred	103,676	39,176
Income tax expense	CAD 109,307	CAD 40,104

Amount of deferred income tax expense (benefit) pertaining to income (loss) from continuing operations.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32639-109319

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08(h))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 6.I.7)
-URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration