Earnings Per Share and Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended		9 Months Ended
	Sep. 18, 2016	Jul. 03, 2016	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Oct. 01, 2015
Statement Of Equity Line Item [Line Items]
Treasury Stock, Shares held			35,000,000		35,000,000
Treasury Stock, Value			$ (2,327)		$ (2,327)		$ (2,327)
Preferred Stock, Value, Issued			$ 0		$ 0		193
Stock Redeemed or Called During Period, Shares	500,000	400,000	600,000		400,000
Preferred Stock, Dividend Rate, Percentage	6.97%	7.125%	6.99%		6.70%
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	4,000,000	1,000,000	2,000,000
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	14,000,000	12,000,000	14,000,000
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			$ 0		$ 0		$ 190	$ 190
Stock Redeemed or Called During Period, Value	$ 90	$ 100