Debt (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Accumulated amortization on debt issuance costs	$ 2,515,000	$ 2,296,000
Company's borrowings
Total notes payable	264,080,000	256,562,000
Less: current portion of long-term debt	9,109,000	5,206,000
Less: Debt issuance costs (net of accumulated amortization of $2,515 and $2,296, respectively	3,817,000	4,105,000
Total long-term debt	$ 251,154,000	$ 247,251,000
Convertible Senior Notes
Line of Credit Facility [Line Items]
Interest rate	2.75%	2.75%
Face amount	$ 143,750,000	$ 143,750,000
Company's borrowings
Total notes payable	127,255,000	126,053,000
REG Geismar GOZone bonds
Company's borrowings
Total notes payable	100,000,000	$ 100,000,000
REG Danville term loan
Line of Credit Facility [Line Items]
Face amount	$ 12,000,000
Description of variable rate basis	LIBOR plus 4% per annum	LIBOR plus 4% per annum
Company's borrowings
Total notes payable	$ 6,192,000	$ 0
REG Danville term loan | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate	4.00%	4.00%
REG Newton term loan
Line of Credit Facility [Line Items]
Description of variable rate basis	LIBOR plus 4% per annum	LIBOR plus 4% per annum
Company's borrowings
Total notes payable	$ 16,106,000	$ 16,800,000
REG Newton term loan | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate	4.00%	4.00%
REG Mason City term loan
Line of Credit Facility [Line Items]
Interest rate	5.00%	5.00%
Company's borrowings
Total notes payable	$ 3,445,000	$ 3,675,000
REG Ames term loans
Company's borrowings
Total notes payable	$ 3,818,000	$ 3,901,000
REG Ames term loans | Minimum
Line of Credit Facility [Line Items]
Interest rate	3.50%	3.50%
REG Ames term loans | Maximum
Line of Credit Facility [Line Items]
Interest rate	4.25%	4.25%
REG Grays Harbor term loan
Line of Credit Facility [Line Items]
Description of variable rate basis	Prime Rate plus 0.25%	Prime Rate plus 0.25%
Basis spread on variable rate	3.50%	3.50%
Company's borrowings
Total notes payable	$ 5,225,000	$ 5,225,000
REG Grays Harbor term loan | Prime Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.25%	0.25%
Other
Company's borrowings
Total notes payable	$ 2,039,000	$ 908,000