Discontinued operations (Details) $ / shares in Units, $ in Millions		3 Months Ended		6 Months Ended
	Jun. 26, 2015 USD ($) item MW	Jun. 30, 2016 USD ($) $ / shares	Jun. 30, 2015 USD ($) $ / shares	Jun. 30, 2016 USD ($) $ / shares	Jun. 30, 2015 USD ($) $ / shares
Discontinued operations
Gain on the disposal					$ (47.3)
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Income from operations of discontinued businesses, net of tax			$ 33.6		21.1
Income from operations of discontinued businesses, net of noncontrolling interests			$ 37.0		$ 32.1
Basic and diluted earnings per share related to income (loss) from discontinued operations (in dollars per share) | $ / shares			$ 0.30		$ 0.26
Wind Projects | Discontinued Operations, Held-for-sale or Disposed of by Sale
Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract]
Revenue			$ 18.1		$ 34.8
Operations and maintenance			5.2		10.8
Depreciation and amortization			0.1		10.3
Total Project expenses			5.3		21.1
Change in fair value of derivatives			6.7		(0.7)
Equity in earnings of unconsolidated affiliates			0.7		(0.2)
Interest expense, net			(3.3)		(6.7)
Gain on sale of asset			47.3		47.3
Total Project other income (expense)			51.4		39.7
Income from operations of discontinued businesses			64.2		53.4
Income tax expense			30.6		32.3
Income from operations of discontinued businesses, net of tax			33.6		21.1
Net loss attributable to noncontrolling interests of discontinued businesses			(3.4)		(11.0)
Income from operations of discontinued businesses, net of noncontrolling interests			$ 37.0		32.1
Basic and diluted earnings per share related to income (loss) from discontinued operations (in dollars per share) | $ / shares		$ 0.30		$ 0.26
Wind Projects | APT | Discontinued Operations, Disposed of by Sale
Discontinued operations
Consideration received on disposal interests	$ 335.0
Wind Projects | Wind (Discontinued Operations) | Discontinued Operations, Held-for-sale or Disposed of by Sale
Net Cash Provided by (Used in) Discontinued Operations [Abstract]
Cash provided by operating activities					21.9
Cash provided by investing activities					$ (12.8)
Wind Projects | Wind (Discontinued Operations) | APT | Discontinued Operations, Disposed of by Sale
Discontinued operations
Ownership interest sold (as a percent)	100.00%
Number of operating wind projects sold | item	5
Wind power project capacity of disposed projects (in MW) | MW	521
Project debt deconsolidated	$ 249.0
Project debt deconsolidated, including adjustment for entities with proportional ownership interest	274.0
Non-controlling interests deconsolidated	$ 224.0
Goshen North | Wind (Discontinued Operations) | APT | Discontinued Operations, Disposed of by Sale
Discontinued operations
Ownership interest sold (as a percent)	12.50%
Idaho Wind | Wind (Discontinued Operations) | APT | Discontinued Operations, Disposed of by Sale
Discontinued operations
Ownership interest sold (as a percent)	27.60%
Meadow Creek | Wind (Discontinued Operations) | APT | Discontinued Operations, Disposed of by Sale
Discontinued operations
Ownership interest sold (as a percent)	100.00%
Rockland Wind Farm | Wind (Discontinued Operations) | APT | Discontinued Operations, Disposed of by Sale
Discontinued operations
Ownership interest sold (as a percent)	50.00%
Consolidated ownership interest sold (as a percent)	100.00%
Canadian Hills | Wind (Discontinued Operations) | APT | Discontinued Operations, Disposed of by Sale
Discontinued operations
Consolidated ownership interest sold (as a percent)	99.00%
Income from Discontinued Operations [Member] | Wind Projects | Wind (Discontinued Operations) | APT | Discontinued Operations, Disposed of by Sale
Discontinued operations
Gain on the disposal		$ 47.0		$ 47.0