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Supplemental Cash Flow Information (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
NON-CASH INVESTING AND FINANCING ACTIVITIES      
Power plant long-term service agreement $ 2.6 $ 39.5 $ 2.3
SUPPLEMENTAL CASH FLOW INFORMATION      
Interest (net of interest capitalized) [1] 139.6 141.9 145.4
Income taxes (net of income tax refunds) (16.0) (5.9) (3.4)
Interest costs capitalized $ 18.0 $ 7.5 $ 4.2
[1] Net of interest capitalized of $18.0 million, $7.5 million and $4.2 million in 2017, 2016 and 2015, respectively.