Debt - (Details) - USD ($)
 1 Months Ended 12 Months Ended
Oct. 31, 2015
Jul. 31, 2015
Dec. 31, 2016
Dec. 31, 2015
Line of Credit Facility [Line Items]        
Total debt before debt issuance costs     $ 217,891,000 $ 256,562,000
Less: Current portion of long-term debt     15,402,000 5,206,000
Less: Debt issuance costs (net of accumulated amortization of $ 3,705 and $2,296, respectively)     6,286,000 4,105,000
Accumulated amortization on debt issuance costs     3,705,000 2,296,000
Total long-term debt     196,203,000 247,251,000
4.00% Convertible Senior Notes, $152,000 face amount, due in June 2036        
Line of Credit Facility [Line Items]        
Total debt before debt issuance costs     $ 113,446,000 0
Interest rate     4.00%  
Face amount     $ 152,000,000  
2.75% Convertible debt, $73,838 face amount, due in June 2019        
Line of Credit Facility [Line Items]        
Total debt before debt issuance costs     $ 67,254,000 126,053,000
Interest rate     2.75%  
Face amount     $ 73,838,000  
REG Geismar GOZone bonds, secured, variable interest rate, due in October 2033        
Line of Credit Facility [Line Items]        
Total debt before debt issuance costs     0 100,000,000
REG Danville term loan, secured, variable interest rate of LIBOR plus 4%, due in December 2017        
Line of Credit Facility [Line Items]        
Total debt before debt issuance costs     $ 8,163,000 0
Face amount $ 12,000,000      
Description of variable rate     LIBOR plus 4% per annum  
REG Danville term loan, secured, variable interest rate of LIBOR plus 4%, due in December 2017 | LIBOR        
Line of Credit Facility [Line Items]        
Variable interest rate 4.00%   4.00%  
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     $ 13,063,000 16,800,000
Description of variable rate     LIBOR plus 4% per annum  
REG Newton term loan, secured, variable interest rate of LIBOR plus 4%, due in December 2018 | LIBOR        
Line of Credit Facility [Line Items]        
Variable interest rate     4.00%  
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     $ 2,659,000 3,675,000
Interest rate     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     $ 3,565,000 3,901,000
REG Ames term loans, secured, fixed interest rates of 3.5% and 4.25%, due in January 2018 and December 2019, respectively | Minimum        
Line of Credit Facility [Line Items]        
Interest rate     3.50%  
REG Ames term loans, secured, fixed interest rates of 3.5% and 4.25%, due in January 2018 and December 2019, respectively | Maximum        
Line of Credit Facility [Line Items]        
Interest rate     4.25%  
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     $ 9,273,000 5,225,000
Description of variable rate     Minimum 3.5% or Prime Rate plus 0.25%  
Variable interest rate   3.50%    
REG Grays Harbor term loan, variable interest of minimum 3.5% or Prime Rate plus 0.25%, due in May 2022 | Prime Rate        
Line of Credit Facility [Line Items]        
Variable interest rate   0.25% 0.25%  
REG Grays Harbor term loan, variable interest of minimum 3.5% or Prime Rate plus 0.25%, due in May 2022 | Minimum        
Line of Credit Facility [Line Items]        
Interest rate     3.50%  
Other        
Line of Credit Facility [Line Items]        
Total debt before debt issuance costs     $ 468,000 $ 908,000