Investment in Constellation Energy Nuclear Group, LLC (CENG) (Details) (CENG, USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Equity in earnings of investments accounted for under the equity method of accounting
Percentage of ownership interest in equity investment (as a percent)	50.01%		50.01%
CENG - equity investment earnings before adjustments for amortization of basis difference	$ 14.9	$ 40.2	$ 27.6	$ 63.6
Amortization of basis difference in CENG	(39.9)	(61.5)	(69.2)	(104.2)
Total equity investment earnings losses-CENG	(25.0)	(21.3)	(41.6)	(40.6)
Summarized income statement information
Revenues	354.5	376.1	713.1	737.0
Expenses	333.8	302.9	675.2	626.2
Income from operations	20.7	73.2	37.9	110.8
Net income	30.9	81.3	57.1	130.4
EDF
Equity in earnings of investments accounted for under the equity method of accounting
Expense of share-based awards	$ 0.5	$ 0.5	$ 0.9	$ 1.6

Amount of amortization of the basis difference in investment in affiliate. The basis difference represents the difference between the fair value of our investment in affiliate less our share of the affiliate's equity because the affiliate's assets and liabilities retained their historical carrying value.

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