Derivative Instruments and Hedging Activities (Notional Volumes) (Details)	Dec. 31, 2013 GWh		Dec. 31, 2012 GWh
Electricity Forwards/Futures, Sales, GWh | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative			3,615
Electricity Forwards/Futures, Sales, GWh | Cash Flow Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Forwards/Futures, Sales, GWh | Cash Flow Hedges | Midwest Generation LLC [Member]
Derivative [Line Items]
Nonmonetary notional amount of derivative			3,615
Electricity Forwards/Futures, Sales, GWh | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	177	[1],[2]	48	[1]
Electricity Forwards/Futures, Sales, GWh | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	7	[2]	47
Electricity Forwards/Futures, Sales, GWh | Economic Hedges | Midwest Generation LLC [Member]
Derivative [Line Items]
Nonmonetary notional amount of derivative	170	[2]	1
Electricity Forwards/Futures, Sales, GWh | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0	[2]	0
Electricity Forwards/Futures, Purchases, GWh | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Forwards/Futures, Purchases, GWh | Cash Flow Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Forwards/Futures, Purchases, GWh | Cash Flow Hedges | Midwest Generation LLC [Member]
Derivative [Line Items]
Nonmonetary notional amount of derivative			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 | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Forwards/Futures, Purchases, GWh | Economic Hedges | Midwest Generation LLC [Member]
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	1		492
Electricity Capacity, Purchases, GW-Day | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Capacity, Purchases, GW-Day | Cash Flow Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Capacity, Purchases, GW-Day | Cash Flow Hedges | Midwest Generation LLC [Member]
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Capacity, Purchases, GW-Day | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Capacity, Purchases, GW-Day | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Capacity, Purchases, GW-Day | Economic Hedges | Midwest Generation LLC [Member]
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Capacity, Purchases, GW-Day | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			60	[3]
Electricity Congestion, Purchases, GWh | Cash Flow Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Congestion, Purchases, GWh | Cash Flow Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Congestion, Purchases, GWh | Cash Flow Hedges | Midwest Generation LLC [Member]
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Electricity Congestion, Purchases, GWh | Economic Hedges
Derivative [Line Items]
Nonmonetary notional amount of derivative	56	[4]	263	[4]
Electricity Congestion, Purchases, GWh | Economic Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	56		263
Electricity Congestion, Purchases, GWh | Economic Hedges | Midwest Generation LLC [Member]
Derivative [Line Items]
Nonmonetary notional amount of derivative	0		0
Electricity Congestion, Purchases, GWh | Trading Activities | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative	262,188	[4]	268,529	[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 | Cash Flow Hedges | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Natural Gas Forwards/Futures, Sales, bcf | Cash Flow Hedges | Midwest Generation LLC [Member]
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 | Other EME Subsidiaries
Derivative [Line Items]
Nonmonetary notional amount of derivative			0
Natural Gas Forwards/Futures, Sales, bcf | Economic Hedges | Midwest Generation LLC [Member]
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			9.9

[1] 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.

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

[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.