Merger and Acquisitions (Details) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended
	Apr. 30, 2011	Apr. 28, 2011
Pending merger with Exelon Corporation
Number of common shares of Constellation Energy to be cancelled and converted into the right to receive for each share of Exelon (in shares)	0.93
Constellation Energy.
Pending merger with Exelon Corporation
Number of common shares of Constellation Energy to be cancelled and converted into the right to receive for each share of Exelon (in shares)	0.93
Termination fee payable under specified circumstances		$ 200
Exelon Corporation
Pending merger with Exelon Corporation
Termination fee payable under specified circumstances		$ 800