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Pension and other post-retirement benefits - Additional Information (Detail) CAD in Thousands, $ in Thousands | 6 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Jun. 30, 2017 USD ($) | Dec. 31, 2017 CAD | Dec. 31, 2017 USD ($) | Dec. 31, 2016 CAD | Dec. 31, 2017 USD ($)
Employee Benefits Disclosure [Line Items] |  |  |  |  |
Defined contribution pension plan cost |  | CAD 9,387 |  | CAD 5,223 |
Accumulated benefit obligation for pension plan |  | 614,840 |  | 317,025 |
Defined benefit plan, amounts recognized in other comprehensive income (loss), net prior service cost (credit) | $ |  |  | $ 2,217 |  |
Net actuarial gain (loss) for defined benefit pension plan that will be amortized from accumulated other comprehensive income over next fiscal year | $ |  |  |  |  | $ 8,204
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) | $ | $ 2,354 |  |  |  |
Gain on curtailment | $ | $ 853 |  |  |  |
Pension Plans |  |  |  |  |
Employee Benefits Disclosure [Line Items] |  |  |  |  |
Expected employer contributions for next year |  | 26,686 |  |  |
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) |  | (49,785) |  | (6,773) |
Gain on curtailment |  | (1,129) |  | 0 |
Other Postretirement Benefit Plans, Defined Benefit |  |  |  |  |
Employee Benefits Disclosure [Line Items] |  |  |  |  |
Expected employer contributions for next year |  | 4,898 |  |  |
Defined Benefit Plan, Benefit Obligation, Actuarial Gain (Loss) |  | (13,321) |  | 2,870 |
Gain on curtailment |  | CAD (6) |  | CAD 0 |

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- Definition Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level. + 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 (e) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan. + 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 (a)(4) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount, before tax, after reclassification adjustment, of (increase) decrease in accumulated other comprehensive income from prior service cost (credit) of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669646-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of (increase) decrease in benefit obligation of defined benefit plan from event reducing expected years of future service of present employees or eliminating accrual of benefits for some or all future services of present employees. + 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 (a)(10) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost (credit) included in accumulated other comprehensive (income) loss expected to be recognized in net periodic benefit cost (credit) for fiscal year following most recent annual statement of financial position. + 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 (s) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of contributions expected to be received by defined benefit plan from employer in next fiscal year following latest fiscal year. + 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 (g) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
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