Shareholder Equity Shareholder Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2015	Jun. 30, 2015	Dec. 31, 2014
Schedule of Capitalization, Equity [Line Items]
Shares Issued, Price Per Share		$ 35
Junior Subordinated Notes, Noncurrent		$ 1,150
Treasury stock, at cost	$ 2,327		$ 2,327
Treasury Stock, Number of Shares Held	35,000,000
Forward Contract Indexed to Issuer's Equity, Indexed Shares		23,000,000
Sale of Stock, Price Per Share		$ 32.48
Temporary Equity, Share Subscriptions		57,500,000
Baltimore Gas and Electric Company [Member]
Schedule of Capitalization, Equity [Line Items]
Preferred Stock, Shares Authorized	6,500,000