Merger (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	Feb. 24, 2011
Purchase price calculation
Shares of Allegheny common stock outstanding	170
Exchange ratio	0.667
Number of shares of FirstEnergy common stock issued	113
Closing price of FirstEnergy common stock	$ 38.16
Fair value of shares issued by FirstEnergy	$ 4,327
Fair value of replacement share-based compensation awards relating to pre-merger service	27
Total consideration transferred	4,354
Fair values of assets and liabilities acquired and related goodwill
Current assets	1,493
Property, plant and equipment	9,656
Investments	138
Goodwill	873
Other noncurrent assets	1,352
Current liabilities	(718)
Noncurrent liabilities	(3,446)
Long-term debt and other long-term obligations	(4,994)
Assets Acquired (Liabilities Assumed), Net, Total	$ 4,354

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Fair Value of the liabilities incurred by the acquirer to former owners of the acquiree, including the fair value of any contingent consideration. If liabilities transferred as consideration remain within the combined entity after the business combination (for example, because the assets or liabilities were transferred to the acquiree rather than to its former owners), and the acquirer therefore retains control of them; in that situation, the acquirer measures those liabilities at their carrying amounts immediately before the acquisition date.

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