Supplemental Cash Flow Information (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
NON-CASH INVESTING AND FINANCING ACTIVITIES
Power plant long-term service agreement		$ 2.3	$ 0.0	$ 9.7
SUPPLEMENTAL CASH FLOW INFORMATION
Interest (net of interest capitalized)	[1]	143.6	143.6	127.5
Income taxes (net of income tax refunds)		(6.2)	0.2	(5.5)
Interest costs capitalized		$ 4.2	$ 2.4	$ 3.4

[1] Net of interest capitalized of $4.2 million, $2.4 million and $3.4 million in 2015, 2014 and 2013, respectively.