Goodwill (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2016 USD ($) item	Sep. 30, 2016 USD ($) GWh
Goodwill
Period of declining power prices	2 years
Goodwill	$ 37.6	$ 134.5
Number of asset groups tested under step one \| item	5
Changes in the carrying amount of goodwill
Goodwill, Beginning Balance		134.5
Impairment of goodwill		(78.8)
Translation Adjustments		(18.1)
Goodwill, Ending Balance	$ 37.6	37.6
Morris And Nipigon
Goodwill
Number of reporting units not significantly impacted by declining power prices \| item	2
Goodwill	$ 6.9	6.9
Changes in the carrying amount of goodwill
Goodwill, Ending Balance	6.9	$ 6.9
Curtis Palmer
Goodwill
Quantity triggering agreement expiration \| GWh		10,000
Expected period of expiration before specified date (in years)		2 years
Moresby Lake
Changes in the carrying amount of goodwill
Impairment of goodwill		$ 0.0
East U.S.
Goodwill
Goodwill	3.3	3.3
Changes in the carrying amount of goodwill
Goodwill, Beginning Balance		3.3
Goodwill, Ending Balance	3.3	3.3
East U.S. \| Curtis Palmer
Goodwill
Goodwill	29.1	44.5
Changes in the carrying amount of goodwill
Goodwill, Beginning Balance		44.5
Impairment of goodwill		(15.4)
Goodwill, Ending Balance	29.1	29.1
Canada \| Nipigon
Goodwill
Goodwill	3.6	3.6
Changes in the carrying amount of goodwill
Goodwill, Beginning Balance		3.6
Goodwill, Ending Balance	3.6	3.6
Canada \| North Bay And Kapuskasing
Goodwill
Long lived asset impairment charge	5.9	5.9
Canada \| North Bay
Goodwill
Goodwill		8.3
Changes in the carrying amount of goodwill
Goodwill, Beginning Balance		8.3
Impairment of goodwill		(6.5)
Translation Adjustments		(1.8)
Canada \| Kapuskasing
Goodwill
Goodwill		8.8
Changes in the carrying amount of goodwill
Goodwill, Beginning Balance		8.8
Impairment of goodwill		(6.7)
Translation Adjustments		(2.1)
Canada \| Mamquam
Goodwill
Goodwill		64.4
Changes in the carrying amount of goodwill
Goodwill, Beginning Balance		64.4
Impairment of goodwill		(50.2)
Translation Adjustments		(14.2)
Canada \| Moresby Lake
Goodwill
Goodwill	1.6	1.6
Changes in the carrying amount of goodwill
Goodwill, Beginning Balance		1.6
Goodwill, Ending Balance	$ 1.6	$ 1.6

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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