Restructuring Activities (Liabilities Subject to Compromise) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 16, 2012	Nov. 15, 2012	Dec. 31, 2011
Liabilities Subject to Compromise
Senior notes, net	$ 3,700
Accounts payable	32
Interest payable	154	38	97
Other	73
Total liabilities subject to compromise	3,959			0
Midwest Generation LLC
Liabilities Subject to Compromise
Accounts payable	29
Interest payable	13
Lease financing	434
Other	53
Total liabilities subject to compromise	$ 529			$ 0