Merger (Details Textuals) (USD $) In Millions, unless otherwise specified	3 Months Ended	7 Months Ended	9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended			3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2011	Sep. 30, 2011	Feb. 24, 2011	Sep. 30, 2011 Allegheny Energy Inc Pre-tax [Member]	Sep. 30, 2010 Allegheny Energy Inc Pre-tax [Member]	Sep. 30, 2011 Allegheny Energy Inc Pre-tax [Member]	Sep. 30, 2010 Allegheny Energy Inc Pre-tax [Member]	Sep. 30, 2011 Allegheny Energy Inc Pre-tax [Member] Other Operating Expense [Member]	Sep. 30, 2010 Allegheny Energy Inc Pre-tax [Member] Other Operating Expense [Member]	Sep. 30, 2011 Allegheny Energy Inc Pre-tax [Member] Other Operating Expense [Member]	Sep. 30, 2010 Allegheny Energy Inc Pre-tax [Member] Other Operating Expense [Member]	Sep. 30, 2011 Allegheny Energy Inc Net of Tax [Member]	Sep. 30, 2011 Allegheny Energy Inc Net of Tax [Member] Other Operating Expense [Member]	Sep. 30, 2010 Allegheny Energy Inc Net of Tax [Member] Other Operating Expense [Member]	Sep. 30, 2011 Allegheny Energy Inc Net of Tax [Member] Other Operating Expense [Member]	Sep. 30, 2010 Allegheny Energy Inc Net of Tax [Member] Other Operating Expense [Member]	Sep. 30, 2011 Pre-tax [Member]	Sep. 30, 2011 Pre-tax [Member]	Sep. 30, 2011 Net of Tax [Member]	Sep. 30, 2011 Net of Tax [Member]
Business Acquisition [Line Items]
Merger related Expenses							$ 56						$ 47
Merger transaction costs					1	33	91	72	2	14	91	35		1	11	73	26
Merger integration costs																		3	88	1	67
Merger (Textuals) [Abstract]
Exchange ratio				0.667
Increase in noncurrent assets			90
Decrease in current assets			16
Decrease in goodwill			79
increase (decrease) In noncurrent liabilities			7
Allegheny's after-tax merger costs	1	57
Expenses related to merger settlements approved by regulatory agencies			33
Inventory valuation adjustment	67	67	67
Inventory valuation adjustment, net of tax	$ 42	$ 42	$ 42