Commitments and Contingencies, Environmental Contingencies - Unrecorded Unconditional Purchase Obligation (Details) $ in Millions	1 Months Ended
	Oct. 31, 2015	Sep. 30, 2015 USD ($)	Apr. 30, 2012 MW	Oct. 30, 2015 Period	Apr. 30, 2014 Issue
Electric Generating Unit Mercury and Air Toxics Standards Rule
Unrecorded Unconditional Purchase Obligation [Line Items]
Generating capacity (in MW) | MW			25
Industrial Boiler Maximum Achievable Control Technology Rules
Unrecorded Unconditional Purchase Obligation [Line Items]
Estimated cost to comply with regulation | $		$ 20
Cross-State Air Pollution Rule
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of issues on which the D.C. Circuit overturned the CSAPR | Issue					2
Subsequent Event | Green House Gas Emission Standard for Existing Sources
Unrecorded Unconditional Purchase Obligation [Line Items]
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 [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of hours measured for standard				8
Current level of air quality concentrations (in parts per billion)				75
Proposed level of air quality concentrations (in parts per billion)				70
Minimum | Electric Generating Unit Mercury and Air Toxics Standards Rule
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of years before affected facilities must demonstrate compliance			3 years
Maximum | Electric Generating Unit Mercury and Air Toxics Standards Rule
Unrecorded Unconditional Purchase Obligation [Line Items]
Number of years before affected facilities must demonstrate compliance			4 years