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Derivatives (Gain Loss) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Derivatives      
Realized gain (loss) on derivatives $ (10.5) $ (17.1) $ 0.3
Gain (loss) on derivative activity (11.1) 9.4 22.1
Commodity Swap      
Derivatives      
Change in fair value of derivatives (20.1) (7.7) 22.4
Realized gain (loss) on derivatives 9.0 17.1 (0.3)
Gain (loss) on derivative activity $ (11.1) $ 9.4 $ 22.1