Finite Lived Intangible Asset and Liability Additional Information (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
	Apr. 21, 2017	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Power Purchase Agreements [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets		$ 0.7	$ 0.7	$ 1.5	$ 1.5
Other Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets		0.1		0.2	$ 0.1
Industrial revenue bond tax savings [Member]
Finite-Lived Intangible Assets [Line Items]
Amortization of Intangible Assets		$ 0.2		$ 0.2
Finite-Lived Intangible Asset, Useful Life	25 years