Supplemental Cash Flow Information (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
NON-CASH INVESTING AND FINANCING ACTIVITIES
Installment payments for Tinker electric distribution system	$ 10.6		$ 0		$ 0
Power plant long-term service agreement	0		1.7		2.7
Future installment payments to wind farm developer	0		0		2.3
SUPPLEMENTAL CASH FLOW INFORMATION
Interest (net of interest capitalized)	161.3	[1]	138.9	[1]	144.6	[1]
Income taxes (net of income tax refunds)	(9.1)		4.7		(139.5)
Interest costs capitalized	$ 8.0		$ 19.1		$ 8.0

[1] Net of interest capitalized of $8.0 million, $19.1 million and $8.0 million in 2012, 2011 and 2010, respectively.