Restructuring Activities (Liabilities Subject to Compromise) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Dec. 31, 2012	Dec. 17, 2012	Nov. 15, 2012
Liabilities Subject to Compromise
Senior notes, net	$ 3,700	$ 3,700	$ 3,700
Accounts payable and accrued liabilities	52	52	32
Interest payable	154	154	154	38	97
Other	73	73	73
Total liabilities subject to compromise	3,979	3,979	3,959
Unpaid contractual interest	70	211
Midwest Generation LLC
Liabilities Subject to Compromise
Lease financing	434	434	434
Accounts payable and accrued liabilities	42	42	29
Interest payable	13	13	13
Other	53	53	53
Total liabilities subject to compromise	542	542	529
Accrued Rejected Contracts [Member] | Midwest Generation LLC
Liabilities Subject to Compromise
Accounts payable and accrued liabilities	$ 32	$ 32