Summary of Significant Accounting Policies - Accounts Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Activity regarding the allowance for doubtful accounts
Balance, beginning of period	$ 1,875	$ 1,351	$ 2,273
Amount charged to selling, general and administrative expenses	139	630	(803)
Charge-offs, net of recoveries	(779)	(106)	(119)
Balance, end of period	$ 1,235	$ 1,875	$ 1,351