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
Investment in Enable		0.0	0.0	1,248.6
SUPPLEMENTAL CASH FLOW INFORMATION
Interest (net of interest capitalized)	[1]	145.4	150.8	151.1
Income taxes (net of income tax refunds)		(3.4)	0.2	(1.1)
Interest costs capitalized		$ 4.2	$ 2.4	$ 5.4

[1] Net of interest capitalized of $4.2 million, $2.4 million and $5.4 million in 2015, 2014 and 2013, respectively.