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Financial instruments - Additional Information (Detail) |  |  | 6 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 24, 2017 USD ($) | Oct. 25, 2016 USD ($) | Jun. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) $ / MWh | Dec. 31, 2017 USD ($) | Jan. 01, 2019 CAD ($) $ / MWh | Jan. 31, 2017 USD ($) | Jan. 31, 2014 USD ($) | Dec. 31, 2012 USD ($)
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Other comprehensive income, foreign currency loss |  |  |  | $ 37,204,000 | $ (17,817,000) |  |  |  |
Other comprehensive income, foreign currency translation adjustment gain (loss) |  |  |  | $ (27,969,000) | (21,753,000) |  |  |  |
Revenue collection period |  |  |  | 45 days |  |  |  |  |
Cash on hand |  |  |  | $ 46,819,000 |  |  |  |  |
Available for drawn on senior debt facilities |  |  |  | 1,046,826,000 |  |  |  |  |
Utility Services |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Accounts receivable balances |  |  |  | 207,740,000 |  |  |  |  |
Liberty Power Group |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Other comprehensive income, foreign currency translation adjustment gain (loss) |  |  |  | 41,244,000 | 19,063,000 |  |  |  |
Bonds | Plan |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, term |  |  | 10 years |  |  |  |  |  |
Long-term debt |  |  | $ 135,000,000 |  |  |  |  |  |
Senior Debt |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  | 479,000,000 |  |  |  |
Par value | $ 750,000,000 |  |  |  |  |  |  |  |
Senior Unsecured Notes |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  | 3,336,795,000 | 3,079,551,000 |  |  |  |
Senior Unsecured Notes | Liberty Power Group | 4.82% Senior Unsecured Notes |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Par value |  |  |  |  |  |  |  |  | $ 150,000,000
Senior Unsecured Notes | Liberty Power Group | 4.65% Senior Unsecured Notes |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Par value |  |  |  |  |  |  |  | $ 200,000,000 |
Senior Unsecured Notes | Liberty Power Group | Convertible Unsecured Subordinated Debentures |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Par value |  |  |  |  |  |  | $ 300,000,000 |  |
Ten Year Foreign Exchange Forward |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Term of forward-starting interest rate swap |  | 10 years |  |  |  |  |  |  |
Notional amount |  | $ 250,000,000 |  |  |  |  |  |  |
Derivative, forward interest rate |  | 1.8395% |  |  |  |  |  |  |
Thirty Year Foreign Exchange Forward |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Term of forward-starting interest rate swap |  | 30 years |  |  |  |  |  |  |
Notional amount |  | $ 250,000,000 |  |  |  |  |  |  |
Derivative, forward interest rate |  | 2.5539% |  |  |  |  |  |  |
Forward contracts |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Other comprehensive income, foreign currency loss |  |  |  | 1,115,000 | $ (297,000) |  |  |  |
Cash flow hedge | Interest rate swaps |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Term of forward-starting interest rate swap |  |  | 10 years |  |  |  |  |  |
Hedge ineffectiveness recognized in earnings |  |  |  | $ 898,000 |  |  |  |  |
Minimum |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Forward price | $ / MWh |  |  |  | 14.55 |  |  |  |  |
Minimum | Senior Debt |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, term | 3 years |  |  |  |  |  |  |  |
Maximum |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Forward price | $ / MWh |  |  |  | 172.97 |  |  |  |  |
Maximum | Senior Debt |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Debt instrument, term | 30 years |  |  |  |  |  |  |  |
Weighted Average |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Forward price | $ / MWh |  |  |  | 24.72 |  |  |  |  |
Non-regulated Energy Sales |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Unrealized gains currently in accumulated other comprehensive loss to be reclassified into net earnings within next twelve months |  |  |  | $ 6,289,000 |  |  |  |  |
Interest expense |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Unrealized gains currently in accumulated other comprehensive loss to be reclassified into net earnings within next twelve months |  |  |  | $ 989,000 |  |  |  |  |
Accounts Receivable [Member] | Credit Concentration Risk [Member] | Liberty Power Group |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Percentage of revenue contributed |  |  |  | 83.98471% |  |  |  |  |
Subsequent Event | Bonds |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Long-term debt |  |  |  |  |  | $ 135,000,000 |  |  |
Subsequent Event | Interest Rate Contract [Member] |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Notional amount |  |  |  |  |  | $ 300,000,000 |  |  |
Derivative, forward interest rate |  |  |  |  |  | 4.60% |  |  |
Minonk Wind Facility | Subsequent Event | Cash flow hedge |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |
Notional quantity (MW-hrs) | $ / MWh |  |  |  |  |  | 251,581,000 |  |  |
Receive average prices (per MW-hr) | $ / MWh |  |  |  |  |  | 20.72 |  |  |

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