Debt and Credit Agreements (Credit Facilities and Letters of Credit) (Details) (USD $) In Millions, unless otherwise specified	Feb. 29, 2012 Revolving credit facility	Dec. 31, 2012 Credit facility due June 2014	Dec. 31, 2012 Letter of Credit	Dec. 31, 2012 Midwest Generation LLC Revolving credit facility	Dec. 31, 2012 Walnut Creek Project Letter of Credit
Line of Credit Facility [Line Items]
Date of termination	February 2012
Line of credit facility	$ 564	$ 100		$ 500
Standby letters of credit			163		30
Standby letters of credit expiring in 2013			91
Standby letters of credit expiring in 2014			2
Standby letters of credit expiring in 2017			21
Standby letters of credit expiring in 2018			18
Standby letters of credit expiring in 2021			18
Standby letters of credit expiring in 2022			13
Cash collateral			$ 49