Derivative Financial Instruments (Details 2) (USD $)	Mar. 31, 2012
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	$ 21,000,000
Mark-to-market energy derivative assets (noncurrent assets)	98,000,000
Total mark-to-market derivative assets	119,000,000
Mark-to-market derivative liabilities (current liabilities)	20,000,000
Mark-to-market energy derivative liabilities (noncurrent liabilities)	34,000,000
Total mark-to-market derivative liabilities	54,000,000
Total mark-to-market derivative net assets (liabilities)	65,000,000
Exelon Generation Co L L C [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	21,000,000
Mark-to-market energy derivative assets (noncurrent assets)	84,000,000
Total mark-to-market derivative assets	105,000,000
Mark-to-market derivative liabilities (current liabilities)	20,000,000
Mark-to-market energy derivative liabilities (noncurrent liabilities)	34,000,000
Total mark-to-market derivative liabilities	54,000,000
Total mark-to-market derivative net assets (liabilities)	51,000,000
Exelon Generation Co L L C [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Mark-to-market energy derivative assets (noncurrent assets)	42,000,000
Total mark-to-market derivative assets	42,000,000
Mark-to-market energy derivative liabilities (noncurrent liabilities)	3,000,000
Total mark-to-market derivative liabilities	3,000,000
Total mark-to-market derivative net assets (liabilities)	39,000,000
Exelon Generation Co L L C [Member] | Derivative [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	3,000,000	[1]
Mark-to-market energy derivative assets (noncurrent assets)	12,000,000	[1]
Total mark-to-market derivative assets	15,000,000	[1]
Mark-to-market derivative liabilities (current liabilities)	2,000,000	[1]
Total mark-to-market derivative liabilities	2,000,000	[1]
Total mark-to-market derivative net assets (liabilities)	13,000,000	[1]
Exelon Generation Co L L C [Member] | Proprietary Trading [Member]
Derivative [Line Items]
Mark-to-market derivative assets (current assets)	18,000,000	[1]
Mark-to-market energy derivative assets (noncurrent assets)	30,000,000	[1]
Total mark-to-market derivative assets	48,000,000	[1]
Mark-to-market derivative liabilities (current liabilities)	18,000,000	[1]
Mark-to-market energy derivative liabilities (noncurrent liabilities)	31,000,000	[1]
Total mark-to-market derivative liabilities	49,000,000	[1]
Total mark-to-market derivative net assets (liabilities)	(1,000,000)	[1]
Other Segment [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Mark-to-market energy derivative assets (noncurrent assets)	14,000,000
Total mark-to-market derivative assets	14,000,000
Total mark-to-market derivative net assets (liabilities)	$ 14,000,000

[1] ) Generation enters into interest rate derivative contracts to economically hedge risk associated with the  interest rate component of commodity positions.  The characterization of the interest rate derivative contracts between the economic hedge and proprietary trading activity in the above table is driven by the corresponding characterization of the underlying commodity position that gives rise to the interest rate exposure.  The company does not utilize interest rate derivatives with the objective of benefiting from shifts or change in market interest rates.