Fair Value of Financial Assets and Liabilities - Fair Value Inputs Assets Quantitative Information (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash collateral excluded	172,000,000		26,000,000
Exelon Generation Co L L C [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities, fair value	293,000,000	[1]	210,000,000	[1]
Exelon Generation Co L L C [Member] | Proprietary Trading [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities, fair value	(15,000,000)	[2],[3]	3,000,000	[2],[4]
Exelon Generation Co L L C [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets, fair value	893,000,000	[2],[3]	488,000,000	[2],[4]
Exelon Generation Co L L C [Member] | Income Approach Valuation Technique [Member] | Minimum [Member] | Proprietary Trading [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward power price assets	15	[2],[5]	10	[2]
Exelon Generation Co L L C [Member] | Income Approach Valuation Technique [Member] | Minimum [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward power price assets	15	[2],[5]	8	[2]
Forward gas price assets	1.52	[2],[5]	2.98	[6]
Exelon Generation Co L L C [Member] | Income Approach Valuation Technique [Member] | Maximum [Member] | Proprietary Trading [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward power price assets	117	[2],[5]	176	[2],[6]
Exelon Generation Co L L C [Member] | Income Approach Valuation Technique [Member] | Maximum [Member] | Proprietary Trading [Member] | Fair Value, Inputs, Level 3 [Member] | All Regions excluding New England [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward power price assets	76
Exelon Generation Co L L C [Member] | Income Approach Valuation Technique [Member] | Maximum [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward power price assets	120	[2],[5]	176	[2],[6]
Forward gas price assets	14.02	[2],[5]	16.63	[6]
Exelon Generation Co L L C [Member] | Income Approach Valuation Technique [Member] | Maximum [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [Member] | All Regions excluding New England [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward power price assets	97		100
Forward gas price assets	8.14		5.7
Exelon Generation Co L L C [Member] | Option Model Valuation Technique [Member] | Minimum [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Volatility percentage	8.00%	[2]	15.00%
Exelon Generation Co L L C [Member] | Option Model Valuation Technique [Member] | Maximum [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Volatility percentage	257.00%	[2]	142.00%
Commonwealth Edison Co [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities, fair value	(207,000,000)	[3]	(193,000,000)	[4]
Commonwealth Edison Co [Member] | Income Approach Valuation Technique [Member] | Minimum [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward heat rate	8.00%	[7]	8.00%	[7]
Marketability Reserve	3.50%		3.50%
Renewable factor	86.00%		84.00%
Commonwealth Edison Co [Member] | Income Approach Valuation Technique [Member] | Maximum [Member] | Derivative [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Forward heat rate	9.00%	[7]	9.00%	[7]
Marketability Reserve	8.00%		8.00%
Renewable factor	126.00%		128.00%

[1] (c)Amount represents the net fair value of out-of-the-money derivative contracts containing credit-risk related contingent features after considering the mitigating effects of offsetting positions under master netting arrangements and reflects the actual net liability upon which any potential contingent collateral obligations would be based.

[2] The valuation techniques, unobservable inputs and ranges are the same for the asset and liability positions.

[3] The fair values do not include cash collateral held on level three positions of $172 million as of December 31, 2014.

[4] The fair values do not include cash collateral held on level three positions of $26 million as of December 31, 2013

[5] The upper ends of the ranges are driven by the winter power and gas prices in the New England region. Without the New England region, the upper ends of the ranges for power and gas would be approximately $97 and $8.14, respectively, and would be approximately $76 for power proprietary trading.

[6] The upper ends of the ranges are driven by the winter power and gas prices in the New England region. Without the New England region, the upper ends of the ranges for power and gas would be approximately $100 and $5.70, respectively.

[7] Quoted forward natural gas rates are utilized to project the forward power curve for the delivery of energy at specified future dates. The natural gas curve is extrapolated beyond its observable period to the end of the contract’s delivery.