Derivative Instruments and Hedging Activities (Notional Volumes) (Details)	Dec. 31, 2012 GWh		Dec. 31, 2011 GWh
Electricity Forwards/Futures, Sales, GWh | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	3,615	[1]	8,320
Electricity Forwards/Futures, Sales, GWh | Cash Flow Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	3,615	[1]	7,978
Electricity Forwards/Futures, Sales, GWh | Cash Flow Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]	342
Electricity Forwards/Futures, Sales, GWh | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	48	[1],[2]	335
Electricity Forwards/Futures, Sales, GWh | Economic Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	1	[1]	227
Electricity Forwards/Futures, Sales, GWh | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	47	[1]	108
Electricity Forwards/Futures, Sales, GWh | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]	0
Electricity Forwards/Futures, Purchases, GWh | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Forwards/Futures, Purchases, GWh | Cash Flow Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Forwards/Futures, Purchases, GWh | Cash Flow Hedges | Other EME Subsidiaries
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 | Economic Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Forwards/Futures, Purchases, GWh | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Forwards/Futures, Purchases, GWh | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	492		2,926
Electricity Capacity, Sales, GW-Day | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative			61	[3]
Electricity Capacity, Sales, GW-Day | Cash Flow Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative			61
Electricity Capacity, Sales, GW-Day | Cash Flow Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Capacity, Sales, GW-Day | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Capacity, Sales, GW-Day | Economic Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Capacity, Sales, GW-Day | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Capacity, Sales, GW-Day | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Capacity, Purchases, GW-Day | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]	0
Electricity Capacity, Purchases, GW-Day | Cash Flow Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]	0
Electricity Capacity, Purchases, GW-Day | Cash Flow Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]	0
Electricity Capacity, Purchases, GW-Day | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]	0
Electricity Capacity, Purchases, GW-Day | Economic Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]	0
Electricity Capacity, Purchases, GW-Day | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]	0
Electricity Capacity, Purchases, GW-Day | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	60	[1],[3]	184
Electricity Congestion, Purchases, GWh | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Congestion, Purchases, GWh | Cash Flow Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Congestion, Purchases, GWh | Cash Flow Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Congestion, Purchases, GWh | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	263	[4]	1,261
Electricity Congestion, Purchases, GWh | Economic Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		608
Electricity Congestion, Purchases, GWh | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	263		653
Electricity Congestion, Purchases, GWh | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	268,529	[4]	230,798
Natural Gas Forwards/Futures, Purchases, bcf | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0
Natural Gas Forwards/Futures, Purchases, bcf | Cash Flow Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0
Natural Gas Forwards/Futures, Purchases, bcf | Cash Flow Hedges | Other EME Subsidiaries
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 | Economic Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0
Natural Gas Forwards/Futures, Purchases, bcf | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0
Natural Gas Forwards/Futures, Purchases, bcf | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	9.9
Natural Gas Forwards/Futures, Sales, bcf | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Natural Gas Forwards/Futures, Sales, bcf | Cash Flow Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Natural Gas Forwards/Futures, Sales, bcf | Cash Flow Hedges | Other EME Subsidiaries
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 | Economic Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Natural Gas Forwards/Futures, Sales, bcf | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Natural Gas Forwards/Futures, Sales, bcf | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0.2
Fuel Oil Forwards/Futures, Purchases, barrels | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Fuel Oil Forwards/Futures, Purchases, barrels | Cash Flow Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Fuel Oil Forwards/Futures, Purchases, barrels | Cash Flow Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Fuel Oil Forwards/Futures, Purchases, barrels | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative			240,000
Fuel Oil Forwards/Futures, Purchases, barrels | Economic Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative			240,000
Fuel Oil Forwards/Futures, Purchases, barrels | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Fuel Oil Forwards/Futures, Purchases, barrels | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0

[1] gigawatt-hours (GWh); gigawatts-day (GW-Day); billion cubic feet (bcf).

[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] 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.