Commitments and Contingencies, Environmental Contingencies - Unrecorded Unconditional Purchase Obligation (Details) $ in Millions	1 Months Ended		6 Months Ended
	Dec. 31, 2014	Apr. 30, 2012 MW	Jun. 30, 2015 USD ($) MW	Apr. 30, 2015 USD ($)	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 [Member]
Environmental Requirements [Abstract]
Generating capacity (in MW) | MW		25
Number of years before affected facilities must demonstrate compliance, low end of range		3 years
Number of years before affected facilities must demonstrate compliance, high end of range		4 years
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 [Member]
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 by the Texas Commission on Environmental Quality			1
Capital Addition Purchase Commitments | Cross-State Air Pollution Rule
Environmental Requirements [Abstract]
Liability for estimated cost to comply with regulation, Maximum			$ 7.0
Capital Addition Purchase Commitments | Regional Haze Rules
Environmental Requirements [Abstract]
Liability for estimated cost to comply with regulation, Maximum			600.0
Estimated annual operating cost to comply with proposed regulation			$ 10.4