Earnings Per Share and Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Statement Of Equity Line Item [Line Items]
Stock options not included in the calculation of diluted common shares outstanding		13,000,000
Treasury Stock, Shares held	35,000,000		35,000,000
Treasury Stock, Value	$ (2,327)		$ (2,327)		$ (2,327)
Preferred Stock, Value, Issued	$ 193		$ 193		193
Equity units issued [Member]
Statement Of Equity Line Item [Line Items]
Stock options not included in the calculation of diluted common shares outstanding	1,000,000	1,000,000	2,000,000	1,000,000
Employee Stock Option [Member]
Statement Of Equity Line Item [Line Items]
Stock options not included in the calculation of diluted common shares outstanding	11,000,000		12,000,000	15,000,000
Baltimore Gas and Electric Company [Member]
Statement Of Equity Line Item [Line Items]
Preferred Stock, Redemption Price Per Share	$ 100		$ 100
Preferred Stock, Value, Issued	$ 190		$ 190		$ 190
Stock Redeemed or Called During Period, Value	100
Preferred Stock, Value, Outstanding	$ 90		$ 90