Commitments and Contingencies, Environmental Contingencies (Details) $ in Millions	3 Months Ended
	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	May. 31, 2015 Parties	Dec. 31, 2014 USD ($)
Regional Haze Rules
Environmental Requirements [Abstract]
Amount spent on installation of emission controls				$ 46.9
Reasonably Attributable Visibility Impairment
Environmental Requirements [Abstract]
Number of environmental advocacy organizations who filed a settlement agreement | Parties			6
Implementation of the National Ambient Air Quality Standard for Sulfur Dioxide
Environmental Requirements [Abstract]
Number of phases under a consent decree which the EPA is requiring states to evaluate areas for attainment	3
Number of months in which the state would have to submit an implementation plan for the respective nonattainment areas	18 months
Number of years for the state to achieve the designated attainment standard	5 years
Fargo MGP Site
Environmental Requirements [Abstract]
Accrual for Environmental Loss Contingencies, Gross	$ 2.2	$ 2.7