Merger (Details Textuals) (USD $) In Millions, unless otherwise specified	10 Months Ended	12 Months Ended		12 Months Ended		3 Months Ended	12 Months Ended						3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2012	Feb. 24, 2011	Dec. 31, 2011 Allegheny Energy Inc	Feb. 24, 2011 Allegheny Energy Inc	Mar. 31, 2011 Allegheny Energy Inc Pre-tax [Member]	Dec. 31, 2012 Allegheny Energy Inc Pre-tax [Member]	Dec. 31, 2011 Allegheny Energy Inc Pre-tax [Member]	Dec. 31, 2010 Allegheny Energy Inc Pre-tax [Member]	Dec. 31, 2012 Allegheny Energy Inc Pre-tax [Member] Other Operating Expense [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, 2012 Allegheny Energy Inc Net of Tax [Member]	Dec. 31, 2011 Allegheny Energy Inc Net of Tax [Member]	Dec. 31, 2012 Allegheny Energy Inc Net of Tax [Member] Other Operating Expense [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]
Merger (Textuals) [Abstract]
Exchange ratio			0.667		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				80
Goodwill					872
Merger transaction costs								91	105	1	91	65				1	73	47
Merger integration costs						67	6	93					42	13	91
Merger related Expenses	58	1
Expenses related to merger settlements approved by regulatory agencies								$ 36