Property, plant and equipment, net - Impairments (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Asset Impairment Charges [Abstract]
Impairment of property, plant and equipment	$ 0.0	$ 4.0
Impairment of Ongoing Project		55.0
Calstock
Asset Impairment Charges [Abstract]
Asset group for testing long-lived assets		7.8
Net working capital		2.3
Impairment of Ongoing Project		4.7
Impairment of intangible assets related to PPAs		0.7
Partial impairment of PP&E		4.0
Impairment losses related to spare parts inventory		1.1
Calstock \| Property, Plant and Equipment
Asset Impairment Charges [Abstract]
Impairment of intangible assets related to PPAs		4.7
Calstock \| Power Purchase agreements
Asset Impairment Charges [Abstract]
Impairment of intangible assets related to PPAs		$ 0.8

Amount of assets in group for testing long-lived assets for impairment.

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- Definition

It represents the amount of net working capital.

+ References

No definition available.

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- References

No definition available.

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- Definition

Amount of impairment loss resulting from write-down of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit to fair value.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16373-109275

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

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- Definition

The aggregate amount of write-downs for impairments recognized during the period for long lived assets held for use (including those held for disposal by means other than sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
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-Section 50
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=109226348&loc=d3e2420-110228

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- Definition

The aggregate amount of write-downs for impairments recognized during the period for long-lived assets held for abandonment, exchange or sale.

+ References

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- Definition

Amount recognized as an operating expense or loss during the period to reduce the carrying amount of a project that has been impaired but not abandoned.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
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-Paragraph 4
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-Publisher FASB
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-Topic 230
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-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

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- Definition

The charge against earnings resulting from the write down of long lived assets other than goodwill due to the difference between the carrying value and lower fair value.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109226691&loc=d3e2921-110230

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-Publisher FASB
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-Topic 230
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-Section 45
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-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

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