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Pension and other post-retirement benefits - Projected Benefit Obligations, Fair Value of Plan Assets, and Funded Status (Detail)
CAD in Thousands, $ in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2017
USD ($)
Dec. 31, 2017
CAD
Dec. 31, 2016
CAD
Change in projected benefit obligation      
Actuarial (gain) loss | $ $ (2,354)    
Gain on curtailment | $ $ 853    
Pension Plans      
Change in projected benefit obligation      
Projected benefit obligation, beginning of year   CAD 331,934 CAD 269,382
Projected benefit obligation assumed from business combination   344,383 63,811
Modifications to pension plan   0 (2,754)
Service cost   17,869 8,435
Interest cost   27,346 13,029
Actuarial (gain) loss   49,785 6,773
Contributions from retirees   0 0
Gain on curtailment   (1,129) 0
Benefits paid   (64,605) (15,845)
Gain on foreign exchange   (48,546) (10,897)
Projected benefit obligation, end of year   657,037 331,934
Change in plan assets      
Fair value of plan assets, beginning of year   236,369 176,171
Plan assets acquired in business combination   247,741 44,258
Actual return on plan assets   82,096 17,221
Employer contributions   38,833 21,776
Benefits paid   (64,605) (15,845)
Loss on foreign exchange   (33,686) (7,212)
Fair value of plan assets, end of year   506,748 236,369
Unfunded status   (150,289) (95,565)
Non-current assets   0 0
Current liabilities   (1,080) (436)
Non-current liabilities   (149,209) (95,129)
Net amount recognized   (150,289) (95,565)
Other Postretirement Benefit Plans, Defined Benefit      
Change in projected benefit obligation      
Projected benefit obligation, beginning of year   83,097 76,565
Projected benefit obligation assumed from business combination   131,263 9,749
Modifications to pension plan   0 (1,235)
Service cost   6,280 2,916
Interest cost   8,621 3,525
Actuarial (gain) loss   13,321 (2,870)
Contributions from retirees   2,364 547
Gain on curtailment   (6) 0
Benefits paid   (8,092) (3,230)
Gain on foreign exchange   (14,834) (2,870)
Projected benefit obligation, end of year   222,014 83,097
Change in plan assets      
Fair value of plan assets, beginning of year   29,139 18,149
Plan assets acquired in business combination   122,900 10,563
Actual return on plan assets   25,612 1,854
Employer contributions   2,683 2,317
Benefits paid   (5,901) (2,683)
Loss on foreign exchange   (10,737) (1,061)
Fair value of plan assets, end of year   163,696 29,139
Unfunded status   (58,318) (53,958)
Non-current assets   4,938 0
Current liabilities   (1,471) (1,242)
Non-current liabilities   (61,785) (52,716)
Net amount recognized   CAD (58,318) CAD (53,958)