Intangible assets and goodwill - Schedule of Intangible Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Cost	$ 101,417	$ 97,520
Accumulated amortization	46,423	46,417
Net book value	54,994	51,103
Power sales contracts
Finite-Lived Intangible Assets [Line Items]
Cost	60,775	56,540
Accumulated amortization	36,063	36,878
Net book value	24,712	19,662
Customer relationships
Finite-Lived Intangible Assets [Line Items]
Cost	26,795	26,799
Accumulated amortization	9,476	8,836
Net book value	17,319	17,963
Interconnection agreements
Finite-Lived Intangible Assets [Line Items]
Cost	13,847	14,181
Accumulated amortization	884	703
Net book value	$ 12,963	$ 13,478

Accumulated amount of amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

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