Supplemental Cash Flow Information (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
NON-CASH INVESTING AND FINANCING ACTIVITIES
Power plant long-term service agreement		$ 28.9	$ (9.2)	$ (2.6)
SUPPLEMENTAL CASH FLOW INFORMATION
Interest (net of interest capitalized)	[1]	144.6	149.7	133.1
Income taxes (net of income tax refunds)		1.3	0.9	(71.5)
Interest costs capitalized		$ 2.8	$ 11.7	$ 18.0

[1] Net of interest capitalized of $2.8 million, $11.7 million and $18.0 million in 2019, 2018 and 2017, respectively.