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Pension and other post-retirement benefits - Amounts Recognized in Accumulated Other Comprehensive Loss (Detail) - CAD CAD in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Plans
Pension and Other Postretirement Benefit Plans, Accumulated Net Gains Losses [Roll Forward]
Beginning balance, January 1	CAD 31,975	CAD 29,461
Additions to AOCI	(3,716)	4,479
Reclassification to regulatory accounts	1,584
Amortization in current period	(1,290)	(1,965)
Ending balance, December 31	28,553	31,975
Pension and Other Post Retirement Benefits Plans, Net Prior Service Cost Credit [Roll Forward]
Beginning balance, January 1	(6,959)	(4,970)
Additions to AOCI	0	(2,754)
Reclassification to regulatory accounts	0
Amortization in current period	868	765
Ending balance, December 31	(6,091)	(6,959)
Actuarial losses (gains) - expected amortization in 2017	(451)
Past service gains - expected amortizations in 2017	781
Other Postretirement Benefit Plans, Defined Benefit
Pension and Other Postretirement Benefit Plans, Accumulated Net Gains Losses [Roll Forward]
Beginning balance, January 1	(5,660)	(2,338)
Additions to AOCI	(4,276)	(3,242)
Reclassification to regulatory accounts	4,902
Amortization in current period	321	(80)
Ending balance, December 31	(4,713)	(5,660)
Pension and Other Post Retirement Benefits Plans, Net Prior Service Cost Credit [Roll Forward]
Beginning balance, January 1	(888)	0
Additions to AOCI	0	(1,235)
Reclassification to regulatory accounts	0
Amortization in current period	365	347
Ending balance, December 31	(523)	CAD (888)
Actuarial losses (gains) - expected amortization in 2017	214
Past service gains - expected amortizations in 2017	CAD 328

X

- Definition

Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment to Regulatory Accounts, before Tax

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Pension and Other Postretirement Benefit Plans, Accumulated Net Gains Losses [Roll Forward]

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Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (j)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580

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Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
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Amount included in accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan expected to be recognized in net periodic benefit (cost) credit for fiscal year following most recent annual statement of financial position.

+ References

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Amount included in accumulated other comprehensive (income) loss for prior service cost (credit) expected to be recognized in net periodic benefit cost (credit) for fiscal year following most recent annual statement of financial position.

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Amount, before tax, of gain (loss) for (increase) decrease in value of benefit obligation for change in actuarial assumptions and increase (decrease) in value of plan assets from experience different from that assumed of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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