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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 |  |

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