Debt (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Company's borrowings
Total notes payable	$ 256,942,000	$ 252,929,000
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	124,861,000	121,354,000
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]
Description of variable rate basis	LIBOR plus 5% per annum	LIBOR plus 5% per annum
Company's borrowings
Total notes payable	$ 313,000	$ 1,513,000
REG Danville term loan | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable rate	5.00%	5.00%
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	$ 17,487,000	$ 19,868,000
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	$ 3,902,000	$ 4,566,000
REG Ames term loans
Company's borrowings
Total notes payable	$ 3,984,000	$ 4,226,000
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	$ 0
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	$ 1,170,000	$ 1,402,000