Commitments and Contingencies (Lease Commitments) (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended	12 Months Ended
	Jan. 02, 2013	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010
Commitments and Guarantees
Capital Leased Assets, Gross		$ 4
Future minimum payments for lease financing:
Operating lease expense		108		110	112
Capital leases, accumulated amortization		(1)
Midwest Generation LLC
Commitments and Guarantees
Capital Leased Assets, Gross		4
Future minimum payments for lease financing:
Operating lease expense		14		16	17
Lease payments not paid	76
Rent paid for sales leaseback transaction	7
Capital leases, accumulated amortization		1
Current portion of lease financings		6		116
Lease financings, net of current portion		2		439
Powerton and Joliet Power Stations
Future minimum payments for operating leases:
Future minimum payments, 2013		151	[1]
Future minimum payments, 2014		151	[1]
Future minimum payments, 2015		67	[1]
Future minimum payments, 2016		26	[1]
Future minimum payments, 2017		1	[1]
Future minimum payments, thereafter		240	[1]
Total future commitments		636	[1]
Other Operating Leases
Future minimum payments for operating leases:
Future minimum payments, 2013		19
Future minimum payments, 2014		17
Future minimum payments, 2015		17
Future minimum payments, 2016		13
Future minimum payments, 2017		15
Future minimum payments, thereafter		130
Total future commitments		211
Other Operating Leases | Midwest Generation LLC
Future minimum payments for operating leases:
Future minimum payments, 2013		8
Future minimum payments, 2014		7
Future minimum payments, 2015		7
Future minimum payments, 2016		5
Future minimum payments, 2017		5
Future minimum payments, thereafter		6
Total future commitments		38
LSTC
Future minimum payments for operating leases:
Future minimum payments, 2013		10
Future minimum payments, 2014		0
Future minimum payments, 2015		0
Future minimum payments, 2016		0
Future minimum payments, 2017		0
Future minimum payments, thereafter		0
Total future commitments		10
LSTC | Midwest Generation LLC
Future minimum payments for lease financing:
Future minimum payments, 2013		129
Future minimum payments, 2014		133
Future minimum payments, 2015		58
Future minimum payments, 2016		23
Future minimum payments, 2017		0
Future minimum payments, thereafter		101
Total future commitments		444
Powerton and Joliet Power Stations | Midwest Generation LLC
Future minimum payments for lease financing:
Future minimum payments, 2013		7
Future minimum payments, 2014		2
Future minimum payments, 2015		0
Future minimum payments, 2016		0
Future minimum payments, 2017		0
Future minimum payments, thereafter		0
Total future commitments		9
Amount representing interest		1
Net commitments		8
Railcars | Midwest Generation LLC
Future minimum payments for lease financing:
Number of units under operating lease		3,200
Number of units under rejected operating lease contract		1,275
Barges | Midwest Generation LLC
Future minimum payments for lease financing:
Number of units under rejected operating lease contract		11
Accrued Liabilities and Other
Future minimum payments for lease financing:
Current portion of lease financings		1
Other Long-Term Liabilities
Future minimum payments for lease financing:
Lease financings, net of current portion		2
Current Portion of Lease Financings | Midwest Generation LLC
Future minimum payments for lease financing:
Current portion of lease financings		1
Lease Financings, Net of Current Portion | Midwest Generation LLC
Future minimum payments for lease financing:
Lease financings, net of current portion		$ 2

[1] Reflects principal and interest payments related to the Powerton and Joliet Sale Leaseback.