Debt - (Details Textual)			1 Months Ended		3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Jun. 02, 2016 USD ($) $ / shares	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($) shares	Sep. 30, 2016 USD ($)	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 08, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jul. 31, 2017 USD ($)	Jun. 30, 2017 USD ($)	Apr. 19, 2017 USD ($)
Short-term Debt [Line Items]
Convertible debt conversion liability	$ 0					$ 0	$ 27,100,000
Notes payable	228,560,000					$ 228,560,000	217,891,000
Share repurchase program, shares repurchased (in shares) | shares						0
Gain on debt extinguishment						$ 0	2,331,000	$ 0
Wells Fargo Revolver
Short-term Debt [Line Items]
Additional increase in maximum borrowing capacity					$ 150,000,000	150,000,000
Line of credit facility limitation amount	40,000,000					$ 40,000,000				$ 50,000,000
Debt instrument, variable rate minimum						0.00%
Percentage required of total current revolving loan commitments						10.00%
Amount required of total current revolving loan commitments						$ 15,000,000
Wells Fargo Revolver | Minimum
Short-term Debt [Line Items]
LIBOR rate margin						1.75%
Fixed charge coverage ratio						100.00%
Wells Fargo Revolver | Maximum
Short-term Debt [Line Items]
LIBOR rate margin						2.25%
Fixed charge coverage ratio						150.00%
BNP Paribas
Short-term Debt [Line Items]
Line of credit	$ 25,000,000					$ 25,000,000
BNP Paribas | Minimum
Short-term Debt [Line Items]
LIBOR rate margin						1.50%
BNP Paribas | Maximum
Short-term Debt [Line Items]
LIBOR rate margin						2.00%
REG Ralston term loan, fixed interest rate of 5%, due in October 2025
Short-term Debt [Line Items]
Face amount													$ 20,000,000
Interest rate	5.00%					5.00%
Notes payable	$ 6,183,000					$ 6,183,000	0
REG Danville Term Loan
Short-term Debt [Line Items]
Face amount											$ 12,500,000
Notes payable	11,460,000					11,460,000	8,163,000
REG Capital term loan, fixed interest rate of 3.99%, due in January 2028
Short-term Debt [Line Items]
Face amount	$ 7,400,000					$ 7,400,000
Interest rate	3.99%					3.99%
Notes payable	$ 7,400,000					$ 7,400,000	$ 0
Prime Rate | REG Ralston term loan, fixed interest rate of 5%, due in October 2025
Short-term Debt [Line Items]
Basis spread on variable rate	5.00%					0.50%
LIBOR | REG Danville Term Loan
Short-term Debt [Line Items]
Basis spread on variable rate	5.38%					4.00%
2.75% Convertible Senior Notes | 2036 Convertible Notes
Short-term Debt [Line Items]
Face amount		$ 152,000,000										$ 152,000,000
Interest rate		4.00%										4.00%
Percentage of convertible senior notes principal required for repurchase		100.00%
Initial conversion rate		92.8074
Initial conversion price (in dollars per share) | $ / shares		$ 10.78
Net proceeds from debt issuance		$ 147,118,000
Fees and offering expenses		4,882,000
Convertible debt conversion liability		40,145,000							$ 45,933,000
Notes payable		111,855,000
Debt discount		$ 40,145,000
Effective interest rate		2.45%
Proceeds from offering costs used to repurchase shares of Common Stock				$ 35,101,000
Share repurchase program, shares repurchased (in shares) | shares				4,060,323
2.75% Convertible Senior Notes | 2019 Convertible Notes
Short-term Debt [Line Items]
Net proceeds from debt issuance			$ 5,584,000		$ 5,584,000							$ 61,954,000
Principal amount of debt repurchased			$ 6,000,000	$ 63,912,000
Gain on debt extinguishment						$ 2,331,000