Summary of Significant Accounting Policies - Accounts Receivable (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Customer receivables	$ 64,598	$ 64,180
Other receivables	2,179	1,466
Rebates receivable	3,027	3,017
Allowance for doubtful accounts	(2,282)	(2,228)
Accounts receivable, net	$ 67,522	$ 66,435	$ 66,400