Merger (Details 2) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended			10 Months Ended		12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Allegheny Energy Inc		Dec. 31, 2012 Allegheny Energy Inc
Revenue and earnings of Allegheny
Revenues	$ 3,500	[1]	$ 4,059	[1]	$ 3,754	[1]	$ 3,990	[1]	$ 3,860	[1]	$ 4,674	[1]	$ 4,037	[1]	$ 3,576	[1]	$ 9,637	$ 10,400	$ 9,820	$ 3,966		$ 4,410
NET INCOME	$ (148)		$ 425		$ 188		$ 306		$ 99		$ 530		$ 193		$ 47		$ 771	$ 869	$ 718	$ 147	[2]	$ 356	[2]
Basic earnings per share of common stock (in dollars per share)	$ (0.35)		$ 1.02		$ 0.45		$ 0.73		$ 0.23		$ 1.27		$ 0.48		$ 0.15		$ 1.85	$ 2.22	$ 2.44	$ 0.37		$ 0.85
Diluted earnings per share of common stock (in dollars per share)	$ (0.35)		$ 1.01		$ 0.45		$ 0.73		$ 0.23		$ 1.27		$ 0.48		$ 0.15		$ 1.84	$ 2.21	$ 2.42	$ 0.37		$ 0.85

[1] Reflects reclassification of revenues and purchased power costs from amounts previously reported to reflect the impact of netting transactions for FES and AE Supply on an hourly basis. This reclassification had no impact on net income or cash flows.

[2] Includes Allegheny’s after-tax merger costs of $58 million and $1 million during 2011 and 2012, respectively.