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Intangible assets and goodwill - Schedule of Intangible Assets (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Finite-Lived Intangible Assets [Line Items] |  |
Cost | $ 168,522 | $ 97,830
Accumulated amortization | 53,609 | 50,214
Net book value | 114,913 | 47,616
Power sales contracts |  |
Finite-Lived Intangible Assets [Line Items] |  |
Cost | 57,943 | 56,206
Accumulated amortization | 41,184 | 38,931
Net book value | 16,759 | 17,275
Customer relationships |  |
Finite-Lived Intangible Assets [Line Items] |  |
Cost | 83,342 | 26,797
Accumulated amortization | 10,967 | 10,104
Net book value | 72,375 | 16,693
Interconnection agreements |  |
Finite-Lived Intangible Assets [Line Items] |  |
Cost | 15,028 | 14,827
Accumulated amortization | 1,458 | 1,179
Net book value | 13,570 | $ 13,648
Other |  |
Finite-Lived Intangible Assets [Line Items] |  |
Cost | 12,209 |
Accumulated amortization | 0 |
Net book value | $ 12,209 |

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