Supplemental Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
NON-CASH INVESTING AND FINANCING ACTIVITIES
Power plant long-term service agreement	$ 1.7		$ 2.7		$ 0
Future installment payments to wind farm developer	0		2.3		3.9
SUPPLEMENTAL CASH FLOW INFORMATION
Interest (net of interest capitalized)	138.9	[1]	144.6	[1]	125.8	[1]
Income taxes (net of income tax refunds)	4.7		(139.5)		2.0
Interest capitalized	$ 19.1		$ 8.0		$ 14.6

[1] (A) Net of interest capitalized of $19.1 million, $8.0 million and $14.6 million in 2011, 2010 and 2009, respectively.