Debt (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Line of Credit Facility [Line Items]
Accumulated amortization on debt issuance costs	$ 877,000	$ 1,139,000
Company's borrowings
Total term debt before debt issuance costs	83,528,000	106,044,000
Less: Current portion of long-term debt	64,367,000	77,131,000
Less: Debt issuance costs (net of accumulated amortization of $877 and $1,139, respectively)	2,353,000	2,783,000
Total long-term debt	$ 16,808,000	26,130,000
4.00% Convertible Senior Notes
Line of Credit Facility [Line Items]
Interest rate	4.00%
Face amount	$ 78,619,000
Company's borrowings
Total term debt before debt issuance costs	$ 61,346,000	69,668,000
REG Danville term loan
Line of Credit Facility [Line Items]
Description of variable rate basis	LIBOR plus 4% per annum
Company's borrowings
Total term debt before debt issuance costs	$ 0	6,468,000
REG Danville term loan | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate	4.00%
REG Ralston term loan
Line of Credit Facility [Line Items]
Description of variable rate basis	Prime Rate plus 2.25%
Company's borrowings
Total term debt before debt issuance costs	$ 15,296,000	15,980,000
REG Ralston term loan | Prime Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	2.25%
REG Grays Harbor term loan
Line of Credit Facility [Line Items]
Description of variable rate basis	Prime Rate plus 0.25%
Basis spread on variable rate	3.50%
Company's borrowings
Total term debt before debt issuance costs	$ 0	6,966,000
REG Grays Harbor term loan | Prime Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.25%
REG Capital term loan
Line of Credit Facility [Line Items]
Interest rate	3.99%
Company's borrowings
Total term debt before debt issuance costs	$ 6,864,000	6,929,000
Other
Company's borrowings
Total term debt before debt issuance costs	$ 22,000	$ 33,000