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Financial instruments - Additional Information (Detail) |  | 3 Months Ended | 6 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Jul. 25, 2018 CAD ($) | Jun. 30, 2020 USD ($) $ / MWh | Jun. 30, 2019 USD ($) | Jun. 30, 2020 USD ($) MWh $ / MWh | Jun. 30, 2019 USD ($) | Dec. 31, 2019 USD ($) $ / MWh | Jul. 17, 2020 CAD ($) | Jul. 17, 2020 USD ($) | Dec. 31, 2019 CAD ($) | Dec. 31, 2019 USD ($) | Sep. 30, 2019 USD ($) instrument | May 23, 2019 USD ($)
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of Megawatt hours | MWh |  |  |  | 116,744 |  |  |  |  |  |  |  |
Cost per Megawatt Hour | $ / MWh |  |  |  |  |  | 38.95 |  |  |  |  |  |
Long-term debt |  | $ 4,154,776,000 |  | $ 4,154,776,000 |  |  |  |  |  | $ 3,931,868,000 |  |
Number of pay-variable and receive-fixed interest rate swaps | instrument |  |  |  |  |  |  |  |  |  |  | 3 |
Realized gain/(loss) in OCI from foreign exchange forward contract |  | 8,573,000 | $ (2,859,000) | (28,057,000) | $ 11,955,000 |  |  |  |  |  |  |
Liberty Power Group |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Realized gain/(loss) in OCI from foreign exchange forward contract |  | 16,642,000 | 3,365,000 | (27,190,000) | 20,205,000 |  |  |  |  |  |  |
Senior Unsecured Notes | Senior Unsecured Notes Due January 2029 |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, term |  |  |  |  |  | 10 years |  |  |  |  |  |
Interest rate (percent) |  |  |  |  |  |  |  |  | 4.60% | 4.60% |  |
Par value |  |  |  |  |  |  |  |  | $ 300,000,000 |  |  |
Senior Unsecured Notes | U.S. Dollar Subordinated Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Long-term debt |  | 621,186,000 |  | 621,186,000 |  |  |  |  |  | $ 621,049,000 |  |
Par value |  | 637,500,000 |  | 637,500,000 |  |  |  |  |  |  | $ 350,000,000 | $ 350,000,000
Senior Unsecured Notes | Liberty Power Group | Convertible Unsecured Subordinated Debentures |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Par value |  | 650,000,000 |  | 650,000,000 |  |  |  |  |  |  |  |
Currency Swap |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Realized gain/(loss) in OCI from foreign exchange forward contract |  | (15,252,000) |  | 19,583,000 |  |  |  |  |  |  |  |
Foreign exchange contract |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Realized gain/(loss) in OCI from foreign exchange forward contract |  | 1,023,000 | $ 9,599,000 | (3,581,000) | $ 24,007,000 |  |  |  |  |  |  |
Non-regulated Energy Sales |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Unrealized gains in AOCI to be reclassified |  |  |  | 8,222,000 |  |  |  |  |  |  |  |
Interest expense |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Unrealized gains in AOCI to be reclassified |  |  |  | 1,395,000 |  |  |  |  |  |  |  |
Sugar Creek |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Amount reclassified from AOCI to earnings |  |  |  |  |  | $ 15,765,000 |  |  |  |  |  |
Canadian Investments and Subsidiaries |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Realized gain/(loss) in OCI from foreign exchange forward contract |  | $ (1,269,000) |  | $ 195,000 |  |  |  |  |  |  |  |
Minimum |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Forward price | $ / MWh |  | 10.53 |  | 10.53 |  |  |  |  |  |  |  |
Minimum | AYES Canada | Measurement Input, Discount Rate |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Alternative investment, measurement input (percent) |  | 0.0798 |  | 0.0798 |  |  |  |  |  |  |  |
Minimum | Atlantica Yield Energy Solutions Canada, Inc | Measurement Input, Price Volatility |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Alternative investment, measurement input (percent) |  | 0.18 |  | 0.18 |  |  |  |  |  |  |  |
Maximum |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Forward price | $ / MWh |  | 147.62 |  | 147.62 |  |  |  |  |  |  |  |
Maximum | AYES Canada | Measurement Input, Discount Rate |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Alternative investment, measurement input (percent) |  | 0.0873 |  | 0.0873 |  |  |  |  |  |  |  |
Maximum | Atlantica Yield Energy Solutions Canada, Inc | Measurement Input, Price Volatility |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Alternative investment, measurement input (percent) |  | 0.22 |  | 0.22 |  |  |  |  |  |  |  |
Weighted Average |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Forward price | $ / MWh |  | 21.95 |  | 21.95 |  |  |  |  |  |  |  |
Weighted Average | AYES Canada | Measurement Input, Discount Rate |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Alternative investment, measurement input (percent) |  | 0.0864 |  | 0.0864 |  |  |  |  |  |  |  |
Subsequent Event | Currency forward contract |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Derivative notional amount |  |  |  |  |  |  | $ 923,243,000 | $ 682,500,000 |  |  |  |
Plan | Bonds |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Debt instrument, term | 10 years |  |  |  |  |  |  |  |  |  |  |
Long-term debt | $ 135,000,000 |  |  |  |  |  |  |  |  |  |  |
Cash flow hedge | Interest rate swaps |  |  |  |  |  |  |  |  |  |  |  |
Derivative [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Term of forward-starting interest rate swap | 10 years |  |  |  |  |  |  |  |  |  |  |

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