Compensation and Benefit Plans (Expense Components) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Pension Plans
Expense components are:
Service cost	$ 14		$ 14		$ 15
Interest cost	13		12		12
Expected return on plan assets	(13)		(12)		(10)
Net amortization	6		6		3
Special termination charges	0		2		0
Total expense	20		22		20
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Net loss (gain)	(43)		6		25
Amortization of net loss	(6)		(6)		(3)
Total in other comprehensive (income) loss	(49)		0		22
Total in expense and other comprehensive (income) loss	(29)		22		42
Estimated prior service cost (credit) that will be reclassified in next fiscal year from accumulated other comprehensive (income) loss	0.2
Estimated net loss that will be reclassified in next fiscal year from accumulated other comprehensive (income) loss	2.1
Pension Plans | Midwest Generation LLC [Member]
Expense components are:
Service cost	12		14		13
Interest cost	9		8		8
Expected return on plan assets	(10)		(9)		(9)
Net amortization	2		2		1
Special termination charges	0		0		0
Total expense	13		15		13
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Net loss (gain)	(36)		4		20
Amortization of net loss	(2)		(2)		(1)
Total in other comprehensive (income) loss	(38)		2		19
Total in expense and other comprehensive (income) loss	(25)		17		32
Estimated prior service cost (credit) that will be reclassified in next fiscal year from accumulated other comprehensive (income) loss	0.2
Estimated net loss that will be reclassified in next fiscal year from accumulated other comprehensive (income) loss	0
Pension Plans | Other EME Subsidiaries
Expense components are:
Service cost	2	[1]	0		2	[1]
Interest cost	4	[1]	4		4	[1]
Expected return on plan assets	(3)	[1]	(3)		(1)	[1]
Net amortization	4	[1]	4		2	[1]
Special termination charges	0	[1]	2		0	[1]
Total expense	7	[1]	7		7	[1]
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Net loss (gain)	(7)	[2]	2	[1]	5	[2]
Amortization of net loss	(4)	[2]	(4)	[1]	(2)	[2]
Total in other comprehensive (income) loss	(11)	[2]	(2)	[1]	3	[2]
Total in expense and other comprehensive (income) loss	(4)	[2]	5	[1]	10	[2]
Postretirement Benefits Other Than Pensions
Expense components are:
Service cost	2		2		2
Interest cost	4		4		4
Net amortization	1		1		0
Total expense	7		7		6
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Net loss (gain)	(10)		(3)		0
Amortization of net loss	(1)		(1)		0
Total in other comprehensive (income) loss	(11)		(4)		0
Total in expense and other comprehensive (income) loss	(4)		3		6
Estimated prior service cost (credit) that will be reclassified in next fiscal year from accumulated other comprehensive (income) loss	0.7
Estimated net loss that will be reclassified in next fiscal year from accumulated other comprehensive (income) loss	0.1
Postretirement Benefits Other Than Pensions | Midwest Generation LLC [Member]
Expense components are:
Service cost	1		1		2
Interest cost	3		3		2
Net amortization	1		1		1
Total expense	5		5		5
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Net loss (gain)	(7)		(3)		1
Amortization of net loss	(1)		(1)		(1)
Total in other comprehensive (income) loss	(8)		(4)		0
Total in expense and other comprehensive (income) loss	(3)		1		5
Estimated prior service cost (credit) that will be reclassified in next fiscal year from accumulated other comprehensive (income) loss	1.0
Estimated net loss that will be reclassified in next fiscal year from accumulated other comprehensive (income) loss	0
Postretirement Benefits Other Than Pensions | Other EME Subsidiaries
Expense components are:
Service cost	1	[1]	1	[1]	0	[1]
Interest cost	1	[1]	1	[1]	2	[1]
Net amortization	0	[1]	0	[1]	(1)	[1]
Total expense	2	[1]	2	[1]	1	[1]
Defined Benefit Plan, Amounts Recognized in Other Comprehensive Income (Loss) [Abstract]
Net loss (gain)	(3)	[1]	0	[1]	(1)	[1]
Amortization of net loss	0	[1]	0	[1]	1	[1]
Total in other comprehensive (income) loss	(3)	[1]	0	[1]	0	[1]
Total in expense and other comprehensive (income) loss	$ (1)	[1]	$ 2	[1]	$ 1	[1]
Equity Securities [Member] | Master Trust Investments | MSCI Index | Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Percentage of performance to benchmark (as a percent)	49.00%		40.00%
Equity Securities [Member] | Master Trust Investments | Russell Index | Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Percentage of performance to benchmark (as a percent)	51.00%		60.00%

[1] Excludes Homer City.

[2] Includes Homer City.