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Financial instruments - Additional Information (Detail) |  | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Oct. 25, 2016 USD ($) | Dec. 31, 2017 CAD MMBTU CAD / MWh | Dec. 31, 2017 USD ($) | Dec. 31, 2016 CAD | Dec. 31, 2017 USD ($) MMBTU CAD / MWh | Jan. 31, 2017 CAD | Jan. 31, 2017 USD ($) | Sep. 29, 2016 CAD | Sep. 29, 2016 USD ($) | Jan. 31, 2014 CAD | Dec. 31, 2012 CAD
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Commodity volumes, Gas | MMBTU |  | 15,457,678 |  |  | 15,457,678 |  |  |  |  |  |
Effective portion of cash flow hedge, gain (loss) |  | CAD 8,714,000 |  | CAD 34,355,000 |  |  |  |  |  |  |
Other comprehensive income, foreign currency loss |  | (21,648,000) |  | 0 |  |  |  |  |  |  |
Other comprehensive income, foreign currency translation adjustment gain (loss) |  | (256,067,000) |  | (67,855,000) |  |  |  |  |  |  |
Issuance of convertible debentures, net of costs |  | 743,881,000 |  | 357,694,000 |  |  |  |  |  |  |
Total realized loss (gain) on derivative financial instruments |  | CAD (16,845,000) |  | 3,289,000 |  |  |  |  |  |  |
Revenue collection period |  | 45 days | 45 days |  |  |  |  |  |  |  |
Cash on hand |  | CAD 54,550,000 |  |  |  |  |  |  |  |  |
Available for drawn on senior debt facilities |  | 1,145,859,000 |  |  |  |  |  |  |  |  |
Utility Services |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Accounts receivable balances | $ |  |  |  |  | $ 204,380,000 |  |  |  |  |  |
Not designated as a hedge |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Gain (loss) on derivative |  | (13,414,000) |  | 20,656,000 |  |  |  |  |  |  |
Total realized loss (gain) on derivative financial instruments |  | (16,950,000) |  | 420,000 |  |  |  |  |  |  |
Liberty Power Group |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income, foreign currency translation adjustment gain (loss) |  | CAD 23,381,000 |  | 6,156,000 |  |  |  |  |  |  |
Bonds |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Par value | $ | $ 500,000,000 |  |  |  |  |  |  |  |  |  |
Bonds | Plan |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Debt instrument, term |  | 10 years | 10 years |  |  |  |  |  |  |  |
Long-term debt |  | CAD 135,000,000 |  |  |  |  |  |  |  |  |
Senior Unsecured Notes |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  | 3,863,296,000 |  | 3,913,415,000 |  |  |  |  |  |  |
Senior Unsecured Notes | Convertible Unsecured Subordinated Debentures |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Issuance of convertible debentures, net of costs | $ |  |  | $ 359,950,000 |  |  |  |  |  |  |  |
Senior Unsecured Notes | Liberty Power Group | 4.82% Senior Unsecured Notes |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Par value |  |  |  |  |  |  |  |  |  |  | CAD 150,000,000
Senior Unsecured Notes | Liberty Power Group | 4.65% Senior Unsecured Notes |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Par value |  |  |  |  |  |  |  |  |  | CAD 200,000,000 |
Senior Unsecured Notes | Liberty Power Group | Convertible Unsecured Subordinated Debentures |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Par value |  |  |  |  |  | CAD 300,000,000 |  |  |  |  |
Foreign exchange forward |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Notional amount |  |  |  |  |  | CAD 744,050,000 | $ 567,665,000 | CAD 47,225,000 | $ 38,400,000 |  |
Foreign exchange forward | Not designated as a hedge |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total realized loss (gain) on derivative financial instruments |  | CAD (16,413,000) |  | 1,371,000 |  |  |  |  |  |  |
Ten Year Foreign Exchange Forward |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Notional amount | $ | $ 250,000,000 |  |  |  |  |  |  |  |  |  |
Term of forward-starting interest rate swap | 10 years |  |  |  |  |  |  |  |  |  |
Derivative, forward interest rate | 1.8395% |  |  |  |  |  |  |  |  |  |
Thirty Year Foreign Exchange Forward |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Notional amount | $ | $ 250,000,000 |  |  |  |  |  |  |  |  |  |
Term of forward-starting interest rate swap | 30 years |  |  |  |  |  |  |  |  |  |
Derivative, forward interest rate | 2.5539% |  |  |  |  |  |  |  |  |  |
Forward contracts |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Commodity volumes, Gas | MMBTU |  | 12,420,000 |  |  | 12,420,000 |  |  |  |  |  |
Other comprehensive income, foreign currency loss |  | CAD 432,000 |  |  |  |  |  |  |  |  |
Interest rate swaps | Not designated as a hedge |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total realized loss (gain) on derivative financial instruments |  | 193,000 |  | 0 |  |  |  |  |  |  |
Energy contracts | Not designated as a hedge |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total realized loss (gain) on derivative financial instruments |  | CAD (730,000) |  | (951,000) |  |  |  |  |  |  |
Cash flow hedge | Foreign exchange forward |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Effective portion of cash flow hedge, gain (loss) | $ |  |  | 718,000 |  |  |  |  |  |  |  |
Gain (loss) on derivative | $ |  |  | $ 36,667,000 |  |  |  |  |  |  |  |
Cash flow hedge | Interest rate swaps |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Term of forward-starting interest rate swap |  | 10 years | 10 years |  |  |  |  |  |  |  |
Effective portion of cash flow hedge, gain (loss) |  | CAD 2,771,000 |  | (3,726,000) |  |  |  |  |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Forward price | CAD / MWh |  | 22.13 |  |  | 22.13 |  |  |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Forward price | CAD / MWh |  | 121.56 |  |  | 121.56 |  |  |  |  |  |
Weighted Average |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Forward price | CAD / MWh |  | 33.20 |  |  | 33.20 |  |  |  |  |  |
Fair Value |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivative financial instruments, assets |  | CAD 87,531,000 |  | 150,870,000 |  |  |  |  |  |  |
Fair Value | Energy contracts | Not designated as a hedge |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivative financial instruments, assets |  | 137,000 |  |  |  |  |  |  |  |  |
Fair Value | Cash flow hedge | Foreign exchange forward | Not designated as a hedge |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivative financial instruments, assets |  |  |  | 17,864,000 |  |  |  |  |  |  |
Carrying amount |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivative financial instruments, assets |  | 87,531,000 |  | 150,870,000 |  |  |  |  |  |  |
Carrying amount | Energy contracts | Not designated as a hedge |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivative financial instruments, assets |  | CAD 137,000 |  |  |  |  |  |  |  |  |
Carrying amount | Cash flow hedge | Foreign exchange forward | Not designated as a hedge |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Derivative financial instruments, assets |  |  |  | 17,864,000 |  |  |  |  |  |  |
Credit Concentration Risk | Accounts Receivable | Liberty Power Group |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Major customer revenue percentage |  | 90.00% | 90.00% |  |  |  |  |  |  |  |
Empire | Forward contracts |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Other comprehensive income, foreign currency loss |  | CAD 16,412,000 |  | CAD (17,684,000) |  |  |  |  |  |  |
Non-regulated Energy Sales [Member] |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Unrealized gains currently in accumulated other comprehensive loss to be reclassified into net earnings within next twelve months |  | 11,612,000 |  |  |  |  |  |  |  |  |
Interest expense |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |
Unrealized gains currently in accumulated other comprehensive loss to be reclassified into net earnings within next twelve months |  | CAD 2,643,000 |  |  |  |  |  |  |  |  |

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