Summary of Significant Accounting Policies - Product Warranty (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Summary of product warranty activity
Beginning balance	$ 2,074	$ 2,280
Charges to expense	2,060	1,042
Costs incurred	(2,218)	(1,248)
Ending balance	$ 1,916	$ 2,074
Minimum
Product Liability Contingency [Line Items]
Limited product warranty period	1 year
Maximum
Product Liability Contingency [Line Items]
Limited product warranty period	2 years