Commitments and Contingencies, Environmental Contingencies (Details) $ in Millions	1 Months Ended		9 Months Ended
	Oct. 31, 2015	Apr. 30, 2012 MW	Sep. 30, 2015 USD ($)	Oct. 30, 2015 Period	Dec. 31, 2010 Kiln Group Boiler
Electric Generating Unit Mercury and Air Toxics Standards Rule
Environmental Requirements [Abstract]
Generating capacity (in MW) | MW		25
Regional Haze Rules
Environmental Requirements [Abstract]
Number of environmental groups who petitioned the U.S. Department of the Interior					2
Number of coal-fired boilers | Boiler					12
Number of coal-fired cement kilns | Kiln					1
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
Capital Addition Purchase Commitments | Regional Haze Rules
Environmental Requirements [Abstract]
Liability for estimated cost to comply with regulation | $			$ 82.4
Subsequent Event | Green House Gas Emission Standard for Existing Sources
Environmental Requirements [Abstract]
Duration for public comment (in days)	90 days
Percentage of a comparable new plant's capital cost which would have to be exceeded to consider a project as a reconstruction under the proposed GHG NSPS for Modified and Reconstructed Power Plants	50.00%
Subsequent Event | National Ambient Air Quality Standards for Ozone
Environmental Requirements [Abstract]
Number of hours measured for standard | Period				8
Current level of air quality concentrations (in parts per billion)				75
Proposed level of air quality concentrations (in parts per billion)				70
Maximum | Electric Generating Unit Mercury and Air Toxics Standards Rule
Environmental Requirements [Abstract]
Number of years before affected facilities must demonstrate compliance		4 years
Minimum | Electric Generating Unit Mercury and Air Toxics Standards Rule
Environmental Requirements [Abstract]
Number of years before affected facilities must demonstrate compliance		3 years