Derivative Instruments and Hedging Activities (Notional Volumes) (Details)	Sep. 30, 2013 GWh		Dec. 31, 2012 GWh
Electricity Forwards/Futures, Sales, GWh | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	908	[1]	3,615
Electricity Forwards/Futures, Sales, GWh | Cash Flow Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	908	[1]	3,615
Electricity Forwards/Futures, Sales, GWh | Cash Flow Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]	0
Electricity Forwards/Futures, Sales, GWh | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	13	[1],[2]	48	[2]
Electricity Forwards/Futures, Sales, GWh | Economic Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]	1
Electricity Forwards/Futures, Sales, GWh | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	13	[1]	47
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	734		492
Electricity Capacity, Purchases, GW-Day | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative			0	[1]
Electricity Capacity, Purchases, GW-Day | Cash Flow Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative			0	[1]
Electricity Capacity, Purchases, GW-Day | Cash Flow Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0	[1]
Electricity Capacity, Purchases, GW-Day | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative			0	[1]
Electricity Capacity, Purchases, GW-Day | Economic Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative			0	[1]
Electricity Capacity, Purchases, GW-Day | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0	[1]
Electricity Capacity, Purchases, GW-Day | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			60	[1],[3]
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	112	[4]	263	[4]
Electricity Congestion, Purchases, GWh | Economic Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Congestion, Purchases, GWh | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	112		263
Electricity Congestion, Purchases, GWh | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	266,795	[4]	268,529	[4]
Natural Gas Forwards/Futures, Sales, bcf | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0	[1]
Natural Gas Forwards/Futures, Sales, bcf | Cash Flow Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0	[1]
Natural Gas Forwards/Futures, Sales, bcf | Cash Flow Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0	[1]
Natural Gas Forwards/Futures, Sales, bcf | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0	[1]
Natural Gas Forwards/Futures, Sales, bcf | Economic Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0	[1]
Natural Gas Forwards/Futures, Sales, bcf | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0	[1]
Natural Gas Forwards/Futures, Sales, bcf | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	13.4		9.9	[1]
Fuel Oil Forwards And Futures, Sales, Barrels of Oil | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0
Fuel Oil Forwards And Futures, Sales, Barrels of Oil | Cash Flow Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0
Fuel Oil Forwards And Futures, Sales, Barrels of Oil | Cash Flow Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0
Fuel Oil Forwards And Futures, Sales, Barrels of Oil | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0
Fuel Oil Forwards And Futures, Sales, Barrels of Oil | Economic Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0
Fuel Oil Forwards And Futures, Sales, Barrels of Oil | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0
Fuel Oil Forwards And Futures, Sales, Barrels of Oil | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	40,000
Weather Forward/Futures, Purchases, CDD | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]
Weather Forward/Futures, Purchases, CDD | Cash Flow Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]
Weather Forward/Futures, Purchases, CDD | Cash Flow Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]
Weather Forward/Futures, Purchases, CDD | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]
Weather Forward/Futures, Purchases, CDD | Economic Hedges | Midwest Generation LLC
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]
Weather Forward/Futures, Purchases, CDD | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[1]
Weather Forward/Futures, Purchases, CDD | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	15,000	[1]

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

[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.