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Commitments and contingencies - Estimates of Future Commitments (Detail)
CAD in Thousands, $ in Thousands
 1 Months Ended 12 Months Ended
Jun. 22, 2017
USD ($)
May 06, 2014
USD ($)
Jan. 31, 2016
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2017
CAD
Commitments Disclosure [Line Items]          
Year 1         CAD 263,772
Year 2         188,441
Year 3         158,641
Year 4         140,628
Year 5         135,237
Thereafter         1,012,025
Total         1,898,744
Power purchase          
Commitments Disclosure [Line Items]          
Year 1         74,025
Year 2         48,344
Year 3         49,940
Year 4         50,214
Year 5         50,495
Thereafter         254,380
Total         527,398
Gas supply and service agreements          
Commitments Disclosure [Line Items]          
Year 1         91,425
Year 2         66,848
Year 3         51,809
Year 4         33,161
Year 5         28,411
Thereafter         97,489
Total         369,143
Service agreements          
Commitments Disclosure [Line Items]          
Year 1         47,695
Year 2         47,211
Year 3         48,529
Year 4         48,827
Year 5         46,548
Thereafter         435,093
Total         673,903
Capital projects          
Commitments Disclosure [Line Items]          
Year 1         41,054
Year 2         17,064
Year 3         65
Year 4         65
Year 5         65
Thereafter         16
Total         58,329
Operating leases          
Commitments Disclosure [Line Items]          
Year 1         9,573
Year 2         8,974
Year 3         8,298
Year 4         8,361
Year 5         9,718
Thereafter         225,047
Total         CAD 269,971
Lawsuit vs. Mountain Water Company and City of Missoula [Member]          
Commitments Disclosure [Line Items]          
Fair value of condemned property | $   $ 88,600      
Litigation settlement, amount | $ $ 83,863        
Gain on long-lived assets | $       $ 4,370  
Mountain Water Company [Member]          
Commitments Disclosure [Line Items]          
Amount permitted for hold-back | $     $ 14,400