Summary of Significant Accounting Policies - Accounts Receivable (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Customer receivables	$ 63,150	$ 59,263
Other receivables	1,128	1,319
Rebates receivable	3,445	1,442
Allowance for doubtful accounts	(2,369)	(1,665)
Accounts receivable, net	$ 65,354	$ 60,359