Derivative Instruments and Hedging Activities (Notional Volumes) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2012		Dec. 31, 2011
Electricity Forwards/Futures, Sales, GWh | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	4,630.0	[1]	8,320.0	[1]
Electricity Forwards/Futures, Sales, GWh | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	116.0	[2]	425.0	[2]
Electricity Forwards/Futures, Sales, GWh | Trading Activities
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Forwards/Futures, Purchases, GWh | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Forwards/Futures, Purchases, GWh | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Forwards/Futures, Purchases, GWh | Trading Activities
Derivative [Line Items]
Nonmonetary notional amount of derivative	2,201.0		2,926.0
Electricity Capacity, Sales, GW-Day | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	37	[3]	89	[3]
Electricity Capacity, Sales, GW-Day | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Capacity, Sales, GW-Day | Trading Activities
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Capacity, Purchases, GW-Day | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Capacity, Purchases, GW-Day | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Capacity, Purchases, GW-Day | Trading Activities
Derivative [Line Items]
Nonmonetary notional amount of derivative	134.0	[3]	184.0	[3]
Electricity Congestion, Purchases, GWh | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Congestion, Purchases, GWh | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	287.0	[4]	2,528.0	[4]
Electricity Congestion, Purchases, GWh | Trading Activities
Derivative [Line Items]
Nonmonetary notional amount of derivative	331,418.0	[4]	230,798.0	[4]
Natural Gas Forwards/Futures, Sales, bcf | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Natural Gas Forwards/Futures, Sales, bcf | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Natural Gas Forwards/Futures, Sales, bcf | Trading Activities
Derivative [Line Items]
Nonmonetary notional amount of derivative			0.2
Natural Gas Forwards/Futures, Purchases, bcf | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0
Natural Gas Forwards/Futures, Purchases, bcf | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0
Natural Gas Forwards/Futures, Purchases, bcf | Trading Activities
Derivative [Line Items]
Nonmonetary notional amount of derivative	1.7
Fuel Oil Forwards/Futures, Purchases, barrels | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Fuel Oil Forwards/Futures, Purchases, barrels | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	240,000.0		240,000.0
Fuel Oil Forwards/Futures, Purchases, barrels | Trading Activities
Derivative [Line Items]
Nonmonetary notional amount of derivative	20,000.0		0
Interest rate contracts
Derivative [Line Items]
Notional amount of derivative	$ 703		$ 644
Floating interest rate received	LIBOR		LIBOR
Derivative, low end of interest rate range (as a percentage)	0.79%		0.79%
Derivative, high end of interest rate range (as a percentage)	4.29%		4.29%
Foward Starting Interest Rate Contracts
Derivative [Line Items]
Notional amount of derivative	$ 641		$ 506
Floating interest rate received	LIBOR		LIBOR
Derivative, low end of interest rate range (as a percentage)	0.7825%
Derivative, high end of interest rate range (as a percentage)	4.0025%
Foward Starting Interest Rate Contracts | Convert Floating Rate Debt to Fixed 3.5429%
Derivative [Line Items]
Derivative, low end of interest rate range (as a percentage)			3.5429%
Foward Starting Interest Rate Contracts | Convert Floating Rate Debt to Fixed 3.57%
Derivative [Line Items]
Derivative, high end of interest rate range (as a percentage)			3.57%
Foward Starting Interest Rate Contracts | Convert Floating Rate Debt to Fixed 4.0025%
Derivative [Line Items]
Derivative, high end of interest rate range (as a percentage)			4.0025%

[1] EME's hedge products include forward and futures contracts that qualify for hedge accounting.

[2] These positions adjust financial and physical positions, or day-ahead and real-time positions, to reduce costs or increase gross margin. The net sales positions of these categories are primarily related to hedge transactions that are not designated as cash flow hedges.

[3] EME's hedge transactions for capacity result from bilateral trades. Capacity sold in the PJM Interconnection, LLC Reliability Pricing Model (PJM RPM) auction is not accounted for as a derivative.

[4] Congestion contracts include financial transmission rights, transmission congestion contracts or congestion revenue rights. These positions are similar to a swap, where the buyer is entitled to receive a stream of revenues (or charges) based on the hourly day-ahead price differences between two locations.