Fair Value - Additional Information (Details)		3 Months Ended		6 Months Ended
	Dec. 15, 2015 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) $ / shares	Jun. 30, 2017 USD ($)	Dec. 31, 2017	Jul. 01, 2017	Sep. 20, 2016 USD ($)	Sep. 10, 2016 USD ($)	Jan. 29, 2016 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash flow hedge gain to be reclassified within 12 months				$ 100,000
Interest rate percentage						8.00%
Not designated as hedging instrument | Natural gas forward contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Gain (loss) on derivative		$ 800,000	$ 900,000	(100,000)	$ (700,000)
Gain on the settlement of contracts		$ 1,200,000	$ 1,100,000	$ 2,300,000	$ 2,200,000
Convertible promissory notes | Convertible Promissory Notes due December 2020, Recourse
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt face amount	$ 160,000,000							$ 75,000,000	$ 75,000,000	$ 25,000,000
Interest rate percentage		6.00%	5.00%	6.00%	5.00%		6.00%
Convertible stock price (in dollars per share) | $ / shares	$ 46.37	$ 46.37		$ 46.37
Conversion ratio	0.75
Probability of likely events | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded derivative liability, measurement input		0.05		0.05
Probability of likely events | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded derivative liability, measurement input		0.35		0.35
Volatility | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded derivative liability, measurement input		0.4		0.4
Volatility | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded derivative liability, measurement input		0.5		0.5
Share Price | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded derivative liability, measurement input | $ / shares		15		15
Share Price | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded derivative liability, measurement input | $ / shares		26		26
Risk free discount rate | Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded derivative liability, measurement input		0.0168		0.0168
Risk free discount rate | Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Embedded derivative liability, measurement input		0.0235		0.0235