Accounts Receivable - Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Allowance for Doubtful Accounts Receivable Roll Forward
Beginning balance, December 31, 2019	$ 8,984
Provision for credit losses	902	$ 721
Write-offs	(287)
Ending balance, March 31, 2020	9,599
Oil & Gas Proppants
Allowance for Doubtful Accounts Receivable Roll Forward
Beginning balance, December 31, 2019	7,640
Provision for credit losses	840
Write-offs	0
Ending balance, March 31, 2020	8,480
Industrial & Specialty Products
Allowance for Doubtful Accounts Receivable Roll Forward
Beginning balance, December 31, 2019	1,344
Provision for credit losses	62
Write-offs	(287)
Ending balance, March 31, 2020	$ 1,119