Commitments and Contingencies, Environmental Contingencies - Unrecorded Unconditional Purchase Obligation (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended
	Dec. 31, 2014	Mar. 31, 2015 MW	Apr. 30, 2014 Issue
Cross-State Air Pollution Rule
Environmental Requirements [Abstract]
Number of issues on which the D.C. Circuit overturned the CSAPR			2
Generating capacity (in MW)		700
Regional Haze Rules [Member]
Environmental Requirements [Abstract]
Number of years to comply with proposed regulation	5 years
Capital Commitments | Cross-State Air Pollution Rule
Environmental Requirements [Abstract]
Liability for estimated cost to comply with regulation		7
Capital Commitments | Regional Haze Rules [Member]
Environmental Requirements [Abstract]
Liability for estimated cost to comply with regulation		600
Estimated annual operating cost to comply with proposed regulation		10.4