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Long-term debt - Schedule of Long-term Debt (Detail) - USD ($) | Dec. 31, 2018 | Oct. 17, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Debt Instrument [Line Items] |  |  |
Less: current portion | $ (13,048,000) |  | $ (12,364,000)
Long-term debt, excluding current portion | 3,323,747,000 |  | 3,067,187,000
Senior Unsecured Notes |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | 3,336,795,000 |  | 3,079,551,000
Less: current portion | (13,048,000) |  | (12,364,000)
Long-term debt, excluding current portion | $ 3,323,747,000 |  | 3,067,187,000
Senior Unsecured Notes | Canadian Dollar Senior Unsecured Notes |  |  |
Debt Instrument [Line Items] |  |  |
Weighted average coupon | 4.43% |  |
Par value | $ 650,669,000 |  |
Long-term debt | $ 474,764,000 |  | 623,223,000
Senior Unsecured Notes | Canadian Dollar Senior Secured Project Notes |  |  |
Debt Instrument [Line Items] |  |  |
Weighted average coupon | 10.25% |  |
Par value | $ 31,310,000 |  |
Long-term debt | $ 22,915,000 |  | 26,709,000
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 | $ 1,218,680,000 |  | 1,217,797,000
Senior Unsecured Notes | U.S. Dollar Senior Unsecured Utility Notes |  |  |
Debt Instrument [Line Items] |  |  |
Weighted average coupon | 5.99% |  |
Par value | $ 222,000,000 |  |
Long-term debt | $ 240,161,000 |  | 246,560,000
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 | $ 676,697,000 |  | 772,871,000
Senior Unsecured Notes | U.S. Dollar Subordinated Unsecured Notes |  |  |
Debt Instrument [Line Items] |  |  |
Weighted average coupon | 6.88% | 6.875% |
Par value | $ 287,500,000 | $ 287,500 |
Long-term debt | $ 278,771,000 |  | 0
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 | $ 97,000,000 |  | 51,827,000
Revolving Credit Facility | Senior Unsecured Notes | Senior Unsecured Bank Credit Facilities |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | 321,807,000 |  | 134,988,000
Revolving Credit Facility | Senior Unsecured Notes | Commercial Paper |  |  |
Debt Instrument [Line Items] |  |  |
Long-term debt | $ 6,000,000 |  | $ 5,576,000

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