Debt - (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Total debt before debt issuance costs	$ 256,562	$ 252,929
Less: Current portion of long-term debt	5,206	5,746
Less: Debt issuance costs (net of accumulated amortization of $ 2,296 and $1,474, respectively)	4,105	5,152
Total long-term debt	247,251	242,031
Accumulated amortization on debt issuance costs	2,296	1,474
2.75% Convertible debt, $143,750 face amount, due in June 2019
Line of Credit Facility [Line Items]
Total debt before debt issuance costs	126,053	121,354
REG Geismar GOZone bonds, secured, variable interest rate, due in October 2033
Line of Credit Facility [Line Items]
Total debt before debt issuance costs	100,000	100,000
REG Danville term loan, secured, variable interest rate of LIBOR plus 5%, due in November 2015
Line of Credit Facility [Line Items]
Total debt before debt issuance costs	0	1,513
REG Newton term loan, secured, variable interest rate of LIBOR plus 4%, due in December 2018
Line of Credit Facility [Line Items]
Total debt before debt issuance costs	16,800	19,868
REG Mason City term loan, fixed interest rate of 5%, due in July 2019
Line of Credit Facility [Line Items]
Total debt before debt issuance costs	$ 3,675	$ 4,566
Interest rate	5.00%	5.00%
REG Ames term loans, secured, fixed interest rates of 3.5% and 4.25%, due in January 2018 and December 2019, respectively
Line of Credit Facility [Line Items]
Total debt before debt issuance costs	$ 3,901	$ 4,226
REG Grays Harbor term loan, variable interest of minimum 3.5% or Prime Rate plus 0.25%, due in May 2022
Line of Credit Facility [Line Items]
Total debt before debt issuance costs	5,225	0
Other
Line of Credit Facility [Line Items]
Total debt before debt issuance costs	$ 908	$ 1,402