Commitments and Contingencies, Environmental Contingencies - Unrecorded Unconditional Purchase Obligation (Details) $ in Millions	1 Months Ended			9 Months Ended
	Oct. 31, 2015	Dec. 31, 2014	Apr. 30, 2012 MW	Sep. 30, 2015 USD ($) MW	Oct. 30, 2015 Period	Apr. 30, 2015 USD ($)	Apr. 30, 2014 Issue
Cross-State Air Pollution Rule
Environmental Requirements [Abstract]
Number of issues on which the D.C. Circuit overturned the CSAPR | Issue							2
Generating capacity (in MW) | MW				700
Electric Generating Unit Mercury and Air Toxics Standards Rule
Environmental Requirements [Abstract]
Generating capacity (in MW) | MW			25
Cost of capital incurred for installed mercury controls						$ 8.0
Regional Haze Rules
Environmental Requirements [Abstract]
Number of years to comply with proposed regulation		5 years
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
Number of units which sulfur dioxide controls could be required				1
Capital Addition Purchase Commitments | Regional Haze Rules
Environmental Requirements [Abstract]
Liability for estimated cost to comply with regulation				$ 600.0
Estimated annual operating cost to comply with proposed regulation				$ 10.4
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