Debt (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Accumulated amortization on debt issuance costs	$ 3,928,000	$ 3,705,000
Company's borrowings
Total term debt before debt issuance costs	217,051,000	217,891,000
Less: Current portion of long-term debt	17,366,000	15,402,000
Less: Debt issuance costs (net of accumulated amortization of $3,928 and $3,705, respectively)	6,065,000	6,286,000
Total long-term debt	$ 193,620,000	196,203,000
4.00% Convertible Senior Notes
Line of Credit Facility [Line Items]
Interest rate	4.00%
Face amount	$ 152,000,000
Company's borrowings
Total term debt before debt issuance costs	$ 114,142,000	113,446,000
2.75% Convertible Senior Notes
Line of Credit Facility [Line Items]
Interest rate	2.75%
Face amount	$ 73,838,000
Company's borrowings
Total term debt before debt issuance costs	$ 67,896,000	67,254,000
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	$ 7,525,000	8,163,000
REG Danville term loan | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate	4.00%
REG Newton 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	$ 12,329,000	13,063,000
REG Newton term loan | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate	4.00%
REG Mason City term loan
Line of Credit Facility [Line Items]
Interest rate	5.00%
Company's borrowings
Total term debt before debt issuance costs	$ 2,416,000	2,659,000
REG Ames term loans
Company's borrowings
Total term debt before debt issuance costs	$ 3,479,000	3,565,000
REG Ames term loans | Minimum
Line of Credit Facility [Line Items]
Interest rate	3.50%
REG Ames term loans | Maximum
Line of Credit Facility [Line Items]
Interest rate	4.25%
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	$ 8,815,000	9,273,000
REG Grays Harbor term loan | Prime Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.25%
Other
Company's borrowings
Total term debt before debt issuance costs	$ 449,000	$ 468,000