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Long-term debt - Schedule of Long-term Debt (Detail) - USD ($) | Dec. 31, 2019 | Oct. 24, 2019 | Jul. 12, 2019 | Jul. 01, 2019 | May 23, 2019 | Dec. 31, 2018 | Oct. 17, 2018
--- | --- | --- | --- | --- | --- | --- | ---
Debt Instrument [Line Items] |  |  |  |  |  |  |
Long-term debt | $ 3,931,868,000 |  |  |  |  | $ 3,336,795,000 |
Less: current portion | (225,013,000) |  |  |  |  | (13,048,000) |
Long-term debt, excluding current portion | $ 3,706,855,000 |  |  |  |  | 3,323,747,000 |
Senior Unsecured Notes | U.S. Dollar Senior Unsecured Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Weighted average coupon | 4.09% |  |  |  |  |  |
Par value | $ 1,225,000,000 |  |  |  |  |  |
Long-term debt | $ 1,219,579,000 |  |  |  |  | 1,218,680,000 |
Senior Unsecured Notes | U.S. Dollar Senior Unsecured Utility Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Weighted average coupon | 6.00% |  |  |  |  |  |
Par value | $ 217,000,000 |  |  |  |  |  |
Long-term debt | $ 233,686,000 |  |  |  |  | 240,161,000 |
Senior Unsecured Notes | U.S Dollar Senior Secured Utility Bonds |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Weighted average coupon | 4.75% |  |  |  |  |  |
Par value | $ 662,500,000 |  |  |  |  |  |
Long-term debt | $ 672,337,000 |  |  |  |  | 676,697,000 |
Senior Unsecured Notes | Canadian Dollar Senior Unsecured Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Weighted average coupon | 4.48% |  |  |  |  |  |
Par value | $ 950,669,000 |  |  |  |  |  |
Long-term debt | $ 728,679,000 |  |  |  |  | 474,764,000 |
Senior Unsecured Notes | Canadian Dollar Senior Secured Project Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Weighted average coupon | 10.22% |  |  |  |  |  |
Par value | $ 28,503,000 |  |  |  |  |  |
Long-term debt | 21,961,000 |  |  |  |  | 22,915,000 |
Senior Unsecured Notes | Senior Unsecured Debt |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Long-term debt | $ 3,310,819,000 |  |  |  |  | 3,058,024,000 |
Senior Unsecured Notes | U.S. Dollar Subordinated Unsecured Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Weighted average coupon | 6.50% |  |  |  |  |  | 6.875%
Par value | $ 637,500,000 |  |  |  | $ 350,000,000 |  | $ 287,500
Long-term debt | $ 621,049,000 |  |  |  |  | 278,771,000 |
Revolving Credit Facility | Senior Unsecured Revolving Credit Facilities |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Par value |  | $ 75,000,000 | $ 500,000,000 |  |  |  |
Revolving Credit Facility | Commercial Paper |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Par value |  |  |  | $ 500,000,000 |  |  |
Revolving Credit Facility | Senior Unsecured Notes |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Weighted average coupon | 0.00% |  |  |  |  |  |
Revolving Credit Facility | Senior Unsecured Notes | Senior Unsecured Revolving Credit Facilities |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Long-term debt | $ 141,577,000 |  |  |  |  | 97,000,000 |
Revolving Credit Facility | Senior Unsecured Notes | Senior Unsecured Bank Credit Facilities |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Long-term debt | 75,000,000 |  |  |  |  | 321,807,000 |
Revolving Credit Facility | Senior Unsecured Notes | Commercial Paper |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Long-term debt | $ 218,000,000 |  |  |  |  | $ 6,000,000 |

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