Restructuring Activities (Details) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended	3 Months Ended		9 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended	0 Months Ended
	Apr. 30, 2013 positions	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Apr. 30, 2013 Midwest Generation LLC positions	Sep. 30, 2013 Midwest Generation LLC	Sep. 30, 2012 Midwest Generation LLC	Sep. 30, 2013 Midwest Generation LLC	Sep. 30, 2012 Midwest Generation LLC	Sep. 30, 2013 Other EME Subsidiaries	Sep. 30, 2013 Other EME Subsidiaries	Nov. 08, 2013 Subsequent Event claims	Nov. 08, 2013 Subsequent Event Midwest Generation LLC claims
Restructuring Cost and Reserve [Line Items]
Number claims received (claims)													1,996	333
Provision for allowable claims		$ 0		$ 19			$ 0		$ 19		$ 0	$ 0
Professional fees		24		80			5		20		19	60
Reorganization items, net		24	7	99	9		5	0	39	0	19	60
Number of positions eliminated	150					120
Severance costs	7					5
Business Acquisition, Future Capital Commitment Limit		350		350
Business Acquisition, Cost of Acquired Entity, Purchase Price		2,635		2,635
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned		350		350
Business Acquisition, Cost of Acquired Entity, Equity Interests Issued and Issuable				12,671,977
Business Acquisition, Termination Fee		$ 65		$ 65