Derivative Financial Instruments - Natural Gas Derivatives (Additional Information) (Details) - Not designated as hedging instrument - Natural gas forward contract - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivative	$ 1.1	$ (0.3)	$ 1.1	$ 2.4
Gain on the settlement of contracts	$ 0.6	$ 1.1	$ 2.9	$ 3.3