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Divestments (Details) $ in Millions							1 Months Ended			3 Months Ended	12 Months Ended
Divestments (Details) $ in Millions	Jun. 26, 2015 USD ($) project MW	Apr. 22, 2015 USD ($) MW	Nov. 05, 2013 USD ($)	Apr. 30, 2013 USD ($)	Apr. 12, 2013 USD ($)	Apr. 02, 2013 USD ($)	Jul. 31, 2014 USD ($)	Mar. 31, 2014 USD ($)	Aug. 31, 2013 USD ($)	Apr. 12, 2013 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Mar. 06, 2014 USD ($)	Jan. 30, 2013 USD ($)	Dec. 31, 2012 USD ($)
Discontinued operations and other divestitures
Gain (loss) from discontinued operations													$ (32.8)
Distributions from unconsolidated affiliates											$ 58.5	$ 76.2	40.9
Repayments of Lines of Credit													67.0
Cadillac and Piedmont Projects
Discontinued operations and other divestitures
Value of asset management contracts assigned			$ 0.5
Piedmont
Discontinued operations and other divestitures
Percentage acquired in entity			2.00%
Final ownership interest (as a percent)			100.00%
Greeley
Discontinued operations and other divestitures
Gain (loss) from discontinued operations								$ 2.1
Consideration received on disposal interests														$ 1.0
Wind Projects \| Idaho Wind
Discontinued operations and other divestitures
Ownership interest sold (as a percent)	27.60%
Wind Projects \| Goshen North [Member]
Discontinued operations and other divestitures
Ownership interest sold (as a percent)	12.50%
Wind Projects \| Meadow Creek
Discontinued operations and other divestitures
Ownership interest sold (as a percent)	100.00%
Wind Projects \| Rockland Wind Farm
Discontinued operations and other divestitures
Ownership interest sold (as a percent)	50.00%
Wind Projects \| Canadian Hills
Discontinued operations and other divestitures
Ownership interest sold (as a percent)	99.00%
Discontinued Operations, Disposed of by Sale \| Greeley
Discontinued operations and other divestitures
Consideration received on disposal interests								$ 1.0
Discontinued Operations, Disposed of by Sale \| Greeley \| Income from Discontinued Operations [Member]
Discontinued operations and other divestitures
Gain (loss) from discontinued operations												$ 2.1
Discontinued Operations, Disposed of by Sale \| Wind Projects
Discontinued operations and other divestitures
Gain (loss) from discontinued operations	$ 46.8
Discontinued Operations, Disposed of by Sale \| Rollcast Energy, Inc.
Discontinued operations and other divestitures
Ownership interest sold (as a percent)			60.00%
Amount paid to minority owner			$ 0.5
Outstanding debt forgiven			1.0
Discontinued Operations, Disposed of by Sale \| Rollcast Energy, Inc. \| Income from Discontinued Operations [Member]
Discontinued operations and other divestitures
Gain (loss) from discontinued operations			$ 1.0
Discontinued Operations, Disposed of by Sale \| Florida Projects [Member]
Discontinued operations and other divestitures
Proceeds from disposal					$ 92.0
Consideration received on disposal interests					117.0					$ 117.0					$ 140.0
Distributions from unconsolidated affiliates										$ 25.0
Repayments of Lines of Credit					$ 64.1
Discontinued Operations, Disposed of by Sale \| Path 15 [Member]
Discontinued operations and other divestitures
Proceeds from disposal				$ 52.0
Consideration received on disposal interests				56.0
Management termination fee				4.0
Equity Method Investment Debt Transferred with Sale				$ 137.2
Discontinued Operations, Disposed of by Sale \| APT \| Idaho Wind
Discontinued operations and other divestitures
Ownership interest sold (as a percent)	27.60%
Discontinued Operations, Disposed of by Sale \| APT \| Goshen North [Member]
Discontinued operations and other divestitures
Ownership interest sold (as a percent)	12.50%
Discontinued Operations, Disposed of by Sale \| APT \| Meadow Creek
Discontinued operations and other divestitures
Ownership interest sold (as a percent)	100.00%
Discontinued Operations, Disposed of by Sale \| APT \| Rockland Wind Farm
Discontinued operations and other divestitures
Ownership interest sold (as a percent)	50.00%
Discontinued Operations, Disposed of by Sale \| APT \| Canadian Hills
Discontinued operations and other divestitures
Ownership interest sold (as a percent)	99.00%
Discontinued Operations, Disposed of by Sale \| APT \| Wind Projects
Discontinued operations and other divestitures
Proceeds from disposal	$ 335.0
Ownership interest sold (as a percent)	100.00%
Number of operating wind projects sold \| project	5
Consideration received on disposal interests	$ 335.0
Capacity of disposed property \| 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
Discontinued Operations, Disposed of by Sale \| APT \| Wind Projects \| Rockland Wind Farm
Discontinued operations and other divestitures
Consolidated ownership interest sold (as a percent)	100.00%
Discontinued Operations, Disposed of by Sale \| APT \| Wind Projects \| Income from Discontinued Operations [Member]
Discontinued operations and other divestitures
Gain (loss) from discontinued operations											$ 46.8
Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Gregory Power Partners, LP
Discontinued operations and other divestitures
Proceeds from disposal						$ 34.6
Consideration received on disposal interests						274.2
Gain on disposal, not discontinued operation									$ 30.4				$ 30.4
Proceeds from sale to be held in escrow account						$ 0.4
Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Delta Person LP [Member]
Discontinued operations and other divestitures
Proceeds from disposal							$ 7.2
Consideration received on disposal interests																$ 37.2
Gain on disposal, not discontinued operation							$ 8.6
Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Ridgeline Energy LLC [Member] \| Frontier Solar LLC [Member]
Discontinued operations and other divestitures
Proceeds from disposal		$ 4.3
Ownership interest sold (as a percent)		100.00%
Capacity of disposed property \| MW		20
Disposal Group, Disposed of by Sale, Not Discontinued Operations \| Ridgeline Energy LLC [Member] \| Frontier Solar LLC [Member] \| Other Income [Member]
Discontinued operations and other divestitures
Gain on disposal, not discontinued operation		$ 2.3