Debt - (Details) - USD ($)		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Jul. 31, 2017	Apr. 19, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Total debt before debt issuance costs	$ 228,560,000	$ 228,560,000			$ 217,891,000
Less: Current portion of long-term debt	13,397,000	13,397,000			15,402,000
Less: Debt issuance costs (net of accumulated amortization of $3,510 and $2,396, respectively)	6,627,000	6,627,000			6,286,000
Accumulated amortization on debt issuance costs	3,510,000	3,510,000			2,296,000
Total long-term debt	208,536,000	208,536,000			196,203,000
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	$ 116,255,000	$ 116,255,000			113,446,000
Interest rate	4.00%	4.00%
Face amount	$ 152,000,000	$ 152,000,000
2.75% Convertible Senior Notes, $73,838 face amount, due in June 2019
Line of Credit Facility [Line Items]
Total debt before debt issuance costs	$ 69,859,000	$ 69,859,000			67,254,000
Interest rate	2.75%	2.75%
Face amount	$ 73,838,000	$ 73,838,000
REG Danville term loan, secured, variable interest rate of LIBOR plus 4%, due in July 2022
Line of Credit Facility [Line Items]
Total debt before debt issuance costs	$ 11,460,000	$ 11,460,000			8,163,000
Face amount			$ 12,500,000
Description of variable rate		LIBOR plus 4% per annum
REG Danville term loan, secured, variable interest rate of LIBOR plus 4%, due in July 2022 | LIBOR
Line of Credit Facility [Line Items]
Variable interest rate	5.38%	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	$ 8,189,000	$ 8,189,000			13,063,000
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	$ 1,153,000	$ 1,153,000			2,659,000
Interest rate	5.00%	5.00%
REG Ralston term loan, fixed interest rate of 5%, due in October 2025
Line of Credit Facility [Line Items]
Total debt before debt issuance costs	$ 6,183,000	$ 6,183,000			0
Interest rate	5.00%	5.00%
Face amount				$ 20,000,000
REG Ralston term loan, fixed interest rate of 5%, due in October 2025 | Prime Rate
Line of Credit Facility [Line Items]
Variable interest rate	5.00%	0.50%
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	$ 0	$ 0			3,565,000
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%	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%	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	$ 7,882,000	$ 7,882,000			9,273,000
Description of variable rate		Minimum 3.5% or Prime Rate plus 0.25%
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%
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%	3.50%
REG Capital term loan, fixed interest rate of 3.99%, due in January 2028
Line of Credit Facility [Line Items]
Total debt before debt issuance costs	$ 7,400,000	$ 7,400,000			0
Interest rate	3.99%	3.99%
Face amount	$ 7,400,000	$ 7,400,000
Other
Line of Credit Facility [Line Items]
Total debt before debt issuance costs	$ 179,000	$ 179,000			$ 468,000