Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Activity regarding the allowance for doubtful accounts

Activity regarding the allowance for doubtful accounts was as follows:

Balance, January 1, 2015	$	2,273
Amount charged to selling, general and administrative expenses	(803		)
Charge-offs, net of recoveries	(119		)
Balance, December 31, 2015	1,351
Amount charged to selling, general and administrative expenses	630
Charge-offs, net of recoveries	(106		)
Balance, December 31, 2016	1,875
Amount charged to selling, general and administrative expenses	139
Charge-offs, net of recoveries	(779		)
Balance, December 31, 2017	$	1,235

Estimated useful lives

Depreciation expense is computed on a straight-line method based upon estimated useful lives of the assets. Estimated useful lives are as follows:

Automobiles and trucks	5 years
Computers and office equipment	5 years
Office furniture and fixtures	7 years
Machinery and equipment	5-30 years
Leasehold improvements	the lesser of the lease term or 30 years
Buildings and improvements	30-40 years

Concentration risk by customer

One customer represented slightly less than 10% of the total consolidated revenues of the Company for the years ended December 31, 2017, 2016 and 2015. All customer amounts disclosed in the table are related to biomass-based diesel sales:

	2017		2016		2015
Customer A	$	182,236	$	144,849	$	114,030