Investment in Constellation Energy Nuclear Group, LLC (CENG) (Details) (CENG, USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Equity in earnings of investments accounted for under the equity method of accounting
Percentage of ownership interest in CENG (as a percent)	50.01%		50.01%
CENG - equity investment earnings before adjustments for amortization of basis difference	$ 79.4	$ 95.1	$ 107.1	$ 158.7
Amortization of basis difference in CENG	(43.7)	(52.4)	(112.9)	(156.6)
Total equity investment earnings (losses) - CENG	35.7	42.7	(5.8)	2.1
Summarized income statement information
Revenues	438.0	456.2	1,151.1	1,193.2
Expenses	283.0	276.2	958.2	902.5
Income from operations	155.0	180.0	192.9	290.7
Net income	159.1	191.1	216.2	321.5
EDF
Equity in earnings of investments accounted for under the equity method of accounting
Expense of share-based awards	$ 0.2	$ 0.5	$ 1.0	$ 2.1