Revenue (Tables)

6 Months Ended

Jun. 30, 2018

Revenue from Contract with Customer [Abstract]

Schedule of Sales by Major Source

The following table disaggregates our sales by major source for the three and six months ended June 30, 2018 (in thousands):

	Three Months Ended   June 30, 2018						Six Months Ended   June 30, 2018
Category	Oil & Gas Proppants		Industrial & Specialty Products		Total Sales		Oil & Gas Proppants		Industrial & Specialty Products		Total Sales
Product	$	242,587	$	103,370	$	345,957	$	481,009	$	159,736	$	640,745
Service	81,476		—		81,476		155,984		17		156,001
Total Sales	$	324,063	$	103,370	$	427,433	$	636,993	$	159,753	$	796,746

Schedule of Changes in Contract Assets and Liabilities

The following tables reflect the changes in our contract assets, which we classify as unbilled receivables and our contract liabilities, which we classify as deferred revenues, for the six months ended June 30, 2018 (in thousands):

	Unbilled Receivables
December 31, 2017	$	5,245
Reclassifications to billed receivables	(8,161		)
Revenues recognized in excess of period billings	5,428
June 30, 2018	$	2,512

	Deferred Revenue
December 31, 2017	$	118,414
Revenues recognized from balances held at the beginning of the period	(20,344		)
Revenues deferred from period collections on unfulfilled performance obligations	13,375
June 30, 2018	$	111,445