Commitments and Contingencies (Legal Matters) (Details)	1 Months Ended							9 Months Ended
	Apr. 30, 2015 project	Aug. 31, 2014 project	May. 31, 2014 project	Jul. 31, 2013 project	Mar. 31, 2013 USD ($) claim	Jul. 31, 2012 USD ($) plaintiff	Apr. 30, 2012 utility	Sep. 30, 2015 USD ($)
Legal Matters - Sierra Club Litigation (Numeric) [Abstract]
Number of separate claims included in the complaint against PPL Montana | claim					39
Average requested amount, per day per violation, of injunctive relief and civil penalties					$ 36,000
Amount of civil penalties to be used for beneficial mitigation projects					$ 100,000
Estimated number of post-2000 projects alleged not to be in compliance with the Clean Air Act | project				40
Number of plant projects that had claims dismissed under alleged Clean Air Act violations | project			3
Number of plant projects included in a second amended complaint under alleged Clean Air Act violations | project		8
Number of plant projects with remaining claims to be pursued by plaintiffs | project	4
Regulatory Issues - Maryland Capacity Order (Numeric) [Abstract]
Number electric utilities ordered to enter into long term contracts | utility							3
Regulatory Issues - Electric - Reliability Standards (Numeric) [Abstract]
Maximum per day penalties for reliability violations								$ 1,000,000
Pacific Northwest Markets
Regulatory Issues - Pacific Northwest Markets (Numeric) [Abstract]
Number of parties claiming refunds | plaintiff						2
Claim sought						$ 23,000,000
Remaining claim outstanding								$ 50,000,000
Settled litigation | Pacific Northwest Markets
Regulatory Issues - Pacific Northwest Markets (Numeric) [Abstract]
Number of parties claiming refunds | plaintiff						1
Settlement amount reached with one of the parties						$ 75,000