Merger (Details Textuals) (USD $) In Millions, unless otherwise specified	12 Months Ended			12 Months Ended		3 Months Ended	12 Months Ended			3 Months Ended	12 Months Ended
	Dec. 31, 2011	Feb. 24, 2011	Dec. 31, 2011 Land Easements And Computer Equipment [Member]	Dec. 31, 2011 Pre-tax [Member]	Dec. 31, 2010 Pre-tax [Member]	Mar. 31, 2011 Allegheny Energy Inc Pre-tax [Member]	Dec. 31, 2011 Allegheny Energy Inc Pre-tax [Member]	Dec. 31, 2011 Allegheny Energy Inc Pre-tax [Member] Other Operating Expense [Member]	Dec. 31, 2010 Allegheny Energy Inc Pre-tax [Member] Other Operating Expense [Member]	Mar. 31, 2011 Allegheny Energy Inc Net of Tax [Member]	Dec. 31, 2011 Allegheny Energy Inc Net of Tax [Member]	Dec. 31, 2011 Allegheny Energy Inc Net of Tax [Member] Other Operating Expense [Member]	Dec. 31, 2010 Allegheny Energy Inc Net of Tax [Member] Other Operating Expense [Member]
Business Acquisition [Line Items]
Merger related Expenses	$ 58
Merger transaction costs				91	105			91	65			73	47
Merger integration costs						67	93			42	91
Merger (Textuals) [Abstract]
Exchange ratio		0.667
Increase in Property Plant and Equipment Due to Business Acquisition Purchase Price Allocation	4
Increase in noncurrent assets	91
Increase (Decrease) in Current Liabilities Due to Business Acquisition Purchase Price Allocation	4
Decrease in current assets	16
Decrease in goodwill	86
increase (decrease) In noncurrent liabilities	(9)
Goodwill		866
Expenses related to merger settlements approved by regulatory agencies	36
Fair value of intangible assets acquired			$ 190