Compensation and Benefit Plans (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Defined Benefit Plan Disclosure [Line Items]
Liability related to after tax value of post retirement benefits to be assumed by parent company	$ 3				$ 3
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions					7
Additional contributions	8				8
Expense components are:
Service cost	2.0		4.0		7.0		8.0
Interest cost	3.0		3.0		6.0		6.0
Expected return on plan assets	(3)		(3)		(6)		(6)
Net amortization	2.0		2.0		3.0		3.0
Total expense	4.0		6.0		10.0		11.0
Pension Plans | Midwest Generation LLC
Defined Benefit Plan Disclosure [Line Items]
Employer contributions					3
Additional contributions	5				5
Expense components are:
Service cost	2.0		3.0		6.0		7.0
Interest cost	2.0		2.0		4.0		4.0
Expected return on plan assets	(2)		(3)		(5)		(5)
Net amortization	0		1.0		1.0		1.0
Total expense	2.0		3.0		6.0		7.0
Pension Plans | Other EME Subsidiaries
Expense components are:
Service cost	0	[1]	1.0	[1]	1.0	[1]	1.0	[1]
Interest cost	1.0	[1]	1.0	[1]	2.0	[1]	2.0	[1]
Expected return on plan assets	(1)	[1]	0	[1]	(1)	[1]	(1)	[1]
Net amortization	2.0	[1]	1.0	[1]	2.0	[1]	2.0	[1]
Total expense	2.0	[1]	3.0	[1]	4.0	[1]	4.0	[1]
Postretirement Benefits Other Than Pensions
Defined Benefit Plan Disclosure [Line Items]
Additional contributions	2				2
Expense components are:
Service cost	0.6		0.6		1.2		1.2
Interest cost	1.0		1.1		2.0		2.2
Net amortization	0.3		0.3		0.6		0.7
Total expense	1.9		2.0		3.8		4.1
Postretirement Benefits Other Than Pensions | Midwest Generation LLC
Defined Benefit Plan Disclosure [Line Items]
Additional contributions	1				1
Expense components are:
Service cost	0.3		0.4		0.6		0.8
Interest cost	0.6		0.7		1.2		1.4
Net amortization	0.4		0.4		0.8		0.8
Total expense	1.3		1.5		2.6		3.0
Postretirement Benefits Other Than Pensions | Other EME Subsidiaries
Expense components are:
Service cost	0.3	[1]	0.2	[1]	0.6	[1]	0.4	[1]
Interest cost	0.4	[1]	0.4	[1]	0.8	[1]	0.8	[1]
Net amortization	(0.1)	[1]	(0.1)	[1]	(0.2)	[1]	(0.1)	[1]
Total expense	$ 0.6	[1]	$ 0.5	[1]	$ 1.2	[1]	$ 1.1	[1]

[1] Excludes Homer City.