Derivative Financial Instruments - Natural Gas Derivatives (Additional Information) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivative	$ 0.4	$ (0.5)	$ (0.7)	$ (1.1)
Not designated as hedging instrument | Natural gas forward contract
Derivative Instruments, Gain (Loss) [Line Items]
Gain (loss) on derivative	(0.1)	(1.1)	(0.7)	(0.7)
Gain on the settlement of contracts	$ 1.5	$ 1.1	$ 2.5	$ 1.6