Restructuring Activities (Liabilities Subject to Compromise) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Dec. 16, 2012	Nov. 15, 2012
Liabilities Subject to Compromise
Senior notes, net	$ 3,700	$ 3,700
Accounts payable and accrued liabilities	55	32
Interest payable	154	154	38	97
Other	72	73
Total liabilities subject to compromise	3,981	3,959
Midwest Generation LLC
Liabilities Subject to Compromise
Accounts payable and accrued liabilities	45	29
Interest payable	13	13
Lease financing	434	434
Other	53	53
Total liabilities subject to compromise	$ 545	$ 529