Income Taxes - AFMC and CBPC - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2009 tax_credit $ / gal	Dec. 31, 2014 USD ($)
AFMC for CBPC Exchange [Line Items]
Number of tax credits | tax_credit					2
Valuation allowance	$ 34,889	$ 21,861
Valuation allowance adjustment	$ 13,028	$ 3,613	$ 3,607
Alternative Fuel Mixture Credit
AFMC for CBPC Exchange [Line Items]
Tax credit amount per gallon (USD per gallon) | $ / gal					0.50
Cellulosic Biofuel Producer Credit
AFMC for CBPC Exchange [Line Items]
Tax credit amount per gallon (USD per gallon) | $ / gal					1.01
Valuation allowance			14,600			$ 13,600
Valuation allowance adjustment			$ 1,000
Exchange of Alternative Fuel Tax Benefit
AFMC for CBPC Exchange [Line Items]
Net tax benefit from AFMC for CBPC exchange				$ 18,800