Related Party Transactions (Notes Receivable From Related Party) (Details) (Midwest Generation LLC, USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Midwest Generation LLC
Related Party Transaction [Line Items]
Due from affiliates, gross	$ 12
Due from affiliates, valuation allowance	(12)
Due from affiliates, carrying value	0	11
Interest receivable from affiliate, gross	55
Interest receivable from affiliate, valuation allowance	(55)
Interest receivable from affiliate, carrying value	0	55
Notes receivable from affiliate, gross	1,311
Notes receivable from affiliate, valuation allowance	(1,311)
Notes receivable from affiliate, carrying value	0	1,323
Total due from affiliates, gross	1,378
Total due from affiliates, valuation allowance	(1,378)
Total due from affiliates, carrying amount	$ 0	$ 1,389