Derivative Financial Instruments - Natural Gas Derivatives (Additional Information) (Details) - Not designated as hedging instrument - Natural gas forward contract - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivative	$ (1.1)	$ 0.8	$ (0.7)	$ (0.1)
Gain on the settlement of contracts	$ 1.1	$ 1.2	$ 1.6	$ 2.3