Debt (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jul. 28, 2017	Apr. 19, 2017
Line of Credit Facility [Line Items]
Accumulated amortization on debt issuance costs	$ 3,633,000	$ 3,510,000
Company's borrowings
Total term debt before debt issuance costs	212,344,000	228,560,000
Less: Current portion of long-term debt	176,746,000	13,397,000
Less: Debt issuance costs (net of accumulated amortization of $3,633 and $3,510, respectively)	4,592,000	6,627,000
Total long-term debt	$ 31,006,000	208,536,000
4.00% Convertible Senior Notes
Line of Credit Facility [Line Items]
Interest rate	4.00%
Face amount	$ 127,500,000
Company's borrowings
Total term debt before debt issuance costs	$ 98,716,000	116,255,000
2.75% Convertible Senior Notes
Line of Credit Facility [Line Items]
Interest rate	2.75%
Face amount	$ 67,257,000
Company's borrowings
Total term debt before debt issuance costs	$ 65,113,000	69,859,000
REG Danville term loan
Line of Credit Facility [Line Items]
Face amount			$ 12,500,000
Description of variable rate basis	LIBOR plus 4% per annum
Company's borrowings
Total term debt before debt issuance costs	$ 10,212,000	11,460,000
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	$ 6,613,000	8,189,000
REG Newton 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]
Face amount				$ 20,000,000
Description of variable rate basis	Prime Rate plus 0.5%
Company's borrowings
Total term debt before debt issuance costs	$ 17,116,000	6,183,000
REG Ralston term loan | Prime Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
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	$ 0	1,153,000
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	$ 7,162,000	7,882,000
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	$ 7,307,000	7,400,000
Other
Company's borrowings
Total term debt before debt issuance costs	$ 105,000	$ 179,000