Significant Accounting Policies (Details 3) (USD $) In Millions, unless otherwise specified			12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Keystone	Dec. 31, 2011 Conemaugh	Dec. 31, 2011 Regulated	Dec. 31, 2011 Generating Assets Y	Dec. 31, 2011 Building and improvements Y	Dec. 31, 2011 Office equipment and furniture Y	Dec. 31, 2011 Transportation equipment Y	Dec. 31, 2011 Computer software Y
Property, plant and equipment:
Percentage ownership interest in electric generating plant (as a percent)			20.99%	10.56%
Net investment in two electric generating plants in which we have an undivided ownership interest	$ 345.2	$ 338.0
Nonregulated generation construction work in progress	$ 162.5	$ 108.3
Depreciation and Depletion Expense
Average annual group straight-line rate of depreciation (as a percent)						3.20%
Average annual group straight-line rate of depreciation for our regulated business (as a percent)					2.80%
Number of years used in individual straight-line method of depreciation for solar generating assets (in years)						30
Estimated Useful Lives - minimum (in years)							3	3	5	3
Estimated Useful Lives - maximum (in years)							50	21	15	15