Compensation and Benefit Plans (Assumptions Used in Calculating Expense) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Weighted-average assumptions used to determine expense:
Discount rate					6.00%
Rate of compensation increase	4.50%
Expected long-term return on plan assets					7.50%
Midwest Generation LLC
Weighted-average assumptions used to determine expense:
Discount rate					6.00%
Rate of compensation increase	4.50%				5.00%
Expected long-term return on plan assets					7.50%
Other EME Subsidiaries
Weighted-average assumptions used to determine expense:
Discount rate					6.00%	[1]
Rate of compensation increase	4.50%	[1]
Expected long-term return on plan assets					7.50%	[1]
Pension Plans
Weighted-average assumptions used to determine expense:
Discount rate	4.50%		5.25%
Expected long-term return on plan assets	7.50%		7.50%
Pension Plans | Midwest Generation LLC
Weighted-average assumptions used to determine expense:
Discount rate	4.50%		5.25%
Rate of compensation increase			5.00%
Expected long-term return on plan assets	7.50%		7.50%
Pension Plans | Other EME Subsidiaries
Weighted-average assumptions used to determine expense:
Discount rate	4.50%	[1]	5.25%	[1]
Expected long-term return on plan assets	7.50%	[1]	7.50%	[1]
Pension Plans | Minimum
Weighted-average assumptions used to determine expense:
Rate of compensation increase			4.50%
Pension Plans | Minimum | Other EME Subsidiaries
Weighted-average assumptions used to determine expense:
Rate of compensation increase			4.50%	[1]	4.50%	[1]
Pension Plans | Maximum
Weighted-average assumptions used to determine expense:
Rate of compensation increase			6.00%
Pension Plans | Maximum | Other EME Subsidiaries
Weighted-average assumptions used to determine expense:
Rate of compensation increase			6.00%	[1]	6.00%	[1]
Postretirement Benefits Other Than Pensions
Weighted-average assumptions used to determine expense:
Discount rate	4.75%		5.50%		6.00%
Assumed health care cost trend rates:
Current year	9.50%		9.75%		8.25%
Ultimate rate	5.30%		5.50%		5.50%
Year ultimate rate reached	2019		2019		2016
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
Effect of a one-percentage-point increase on accumulated benefit obligation	$ 18
Effect of a one-percentage-point increase on total of service and interest cost components	1
Effect of a one-percentage-point decrease on accumulated benefit obligation	16
Effect of a one-percentage-point decrease on total of service and interest cost components	1
Postretirement Benefits Other Than Pensions | Midwest Generation LLC
Weighted-average assumptions used to determine expense:
Discount rate	4.75%		5.50%		6.00%
Assumed health care cost trend rates:
Current year	9.50%		9.75%		8.25%
Ultimate rate	5.25%		5.50%		5.50%
Year ultimate rate reached	2019		2019		2016
Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates
Effect of a one-percentage-point increase on accumulated benefit obligation	9
Effect of a one-percentage-point increase on total of service and interest cost components	1
Effect of a one-percentage-point decrease on accumulated benefit obligation	8
Effect of a one-percentage-point decrease on total of service and interest cost components	$ 1
Postretirement Benefits Other Than Pensions | Other EME Subsidiaries
Weighted-average assumptions used to determine expense:
Discount rate	4.75%	[1]	5.50%	[1]	6.00%	[1]
Assumed health care cost trend rates:
Current year	9.50%	[1]	9.75%	[1]	8.25%	[1]
Ultimate rate	5.30%	[1]	5.50%	[1]	5.50%	[1]
Year ultimate rate reached	2019	[1]	2019	[1]	2016	[1]

[1] Includes Homer City.