Pension and other post-retirement benefits - Amounts Recognized in Accumulated Other Comprehensive Loss (Detail) - CAD CAD in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Pension Plans
Pension and Other Postretirement Benefit Plans, Accumulated Net Gains Losses [Roll Forward]
Beginning balance, January 1	CAD 29,461	CAD 29,314
Additions to AOCI	4,479	1,505
Amortization in current period	(1,965)	(1,358)
Ending balance, December 31	31,975	29,461
Pension and Other Post Retirement Benefits Plans, Net Prior Service Cost Credit [Roll Forward]
Beginning balance, January 1	(4,970)	(563)
Additions to AOCI	(2,754)	(4,864)
Amortization in current period	765	457
Ending balance, December 31	(6,959)	(4,970)
Actuarial losses (gains) - expected amortization in 2017	(1,437)
Past service gains - expected amortizations in 2017	836
Other Postretirement Benefit Plans, Defined Benefit
Pension and Other Postretirement Benefit Plans, Accumulated Net Gains Losses [Roll Forward]
Beginning balance, January 1	(2,338)	5,896
Additions to AOCI	(3,242)	(7,554)
Amortization in current period	(80)	(680)
Ending balance, December 31	(5,660)	(2,338)
Pension and Other Post Retirement Benefits Plans, Net Prior Service Cost Credit [Roll Forward]
Beginning balance, January 1	0	0
Additions to AOCI	(1,235)	0
Amortization in current period	347	0
Ending balance, December 31	(888)	CAD 0
Actuarial losses (gains) - expected amortization in 2017	545
Past service gains - expected amortizations in 2017	CAD 0

Pension and Other Post Retirement Benefits Plans, Net Prior Service Cost Credit [Roll Forward]

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- Definition

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

+ References

No definition available.

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- Definition

The pretax net amount of gains and losses that are not yet recognized as a component of net periodic benefit cost, and that are recognized as increases or decreases in other comprehensive income as they arise. Gains and losses are due to changes in the value of either the benefit obligation or the plan assets resulting from experience different from that assumed or from a change in an actuarial assumption, or the consequence of a decision to temporarily deviate from the substantive plan.

+ References

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- Definition

Amount related to the pretax cost of benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation, which has not yet been recognized as components of net periodic benefit cost.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Prior Service Cost
-URI http://asc.fasb.org/extlink&oid=49177896

Reference 2: 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=65897934&loc=d3e1928-114920

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (j)
-URI http://asc.fasb.org/extlink&oid=66004124&loc=SL7669646-108580

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- Definition

Amount included in accumulated other comprehensive income (AOCI) related to gain (loss) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position.

+ References

No definition available.

+ Details

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- Definition

Amount included in accumulated other comprehensive income (AOCI) related to prior service cost (credit) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position.

+ References

No definition available.

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- Definition

Amount before tax of cost (credit) of benefit changes attributable to plan participants' prior service resulting from a plan amendment or plan initiation 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 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (j)
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-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=49177896

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- Definition

Amount before tax of reclassification adjustment from accumulated other comprehensive (income) loss for prior service cost (credit) related to pension and other postretirement benefit plans.

+ 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)
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- Definition

Amount before tax of (increase) decrease in the value of the projected benefit obligation related to a change in an actuarial assumption and increase (decrease) in the value of the plan assets resulting from experience different from that assumed, 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 220
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=66004124&loc=d3e637-108580

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Amount before tax of reclassification adjustment from accumulated other comprehensive income (loss) for actuarial gain (loss) related to pension and other postretirement benefit plans.

+ References

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-Publisher FASB
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