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Intangible assets and goodwill - Schedule of Intangible Assets (Detail) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Finite-Lived Intangible Assets [Line Items] |  |
Cost | $ 97,830 | $ 101,417
Accumulated amortization | 50,214 | 46,423
Net book value | 47,616 | 54,994
Power sales contracts |  |
Finite-Lived Intangible Assets [Line Items] |  |
Cost | 56,206 | 60,775
Accumulated amortization | 38,931 | 36,063
Net book value | 17,275 | 24,712
Customer relationships |  |
Finite-Lived Intangible Assets [Line Items] |  |
Cost | 26,797 | 26,795
Accumulated amortization | 10,104 | 9,476
Net book value | 16,693 | 17,319
Interconnection agreements |  |
Finite-Lived Intangible Assets [Line Items] |  |
Cost | 14,827 | 13,847
Accumulated amortization | 1,179 | 884
Net book value | $ 13,648 | $ 12,963

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