Mergers, Acquisitions, and Dispositions - Assets Disposition Table (Details) - USD ($)	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Assets Disposition Table [Line Items]
Payments to Acquire Businesses, Gross	$ 6,642,000,000	$ 28,000,000
Other current liabilities	1,025,000,000		$ 842,000,000
Pepco Holdings [Member]
Assets Disposition Table [Line Items]
Payments to Acquire Businesses, Gross	7,142,000,000
Potomac Electric Power Company [Member]
Assets Disposition Table [Line Items]
Gain (Loss) on Disposition of Property Plant Equipment	8,000,000
Other current liabilities	$ 13,000,000		$ 25,000,000