Debt - (Details) - USD ($)		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Jun. 15, 2019	Dec. 31, 2018	Oct. 19, 2018	Jul. 31, 2017	Apr. 19, 2017
Line of Credit Facility [Line Items]
Total debt before debt issuance costs		$ 106,044,000		$ 185,817,000
Less: Current portion of long-term debt		77,131,000		149,006,000
Less: Debt issuance costs (net of accumulated amortization of $1,139 and $3,873, respectively)		2,783,000		3,390,000
Total long-term debt		26,130,000		33,421,000
Accumulated amortization on debt issuance costs		1,139,000		3,873,000
2.75% Convertible Senior Notes, matured and paid in June 2019
Line of Credit Facility [Line Items]
Total debt before debt issuance costs		$ 0		66,361,000
Interest rate		2.75%
Face amount			$ 67,380,000
4.00% Convertible Senior Notes, $89,627 face amount, due in June 2036
Line of Credit Facility [Line Items]
Total debt before debt issuance costs		$ 69,668,000		75,477,000
Interest rate		4.00%
Face amount		$ 89,627,000
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		$ 6,468,000		8,964,000
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		4.00%
REG Ralston term loan, variable interest rate of LIBOR plus 2.25%, due in October 2025
Line of Credit Facility [Line Items]
Total debt before debt issuance costs		$ 15,980,000		18,948,000
Interest rate		2.25%
Face amount					$ 19,177,000		$ 20,000,000
REG Ralston term loan, variable interest rate of LIBOR plus 2.25%, due in October 2025 | Prime Rate
Line of Credit Facility [Line Items]
Variable interest rate	3.96%	2.25%
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		$ 6,966,000		8,828,000
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%
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		$ 6,929,000		7,185,000
Interest rate		3.99%
Face amount		$ 7,400,000
Other
Line of Credit Facility [Line Items]
Total debt before debt issuance costs		$ 33,000		$ 54,000