Revenue Recognition - Comparison to Amounts if ASC 605 Had Been in Effect (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Comparison to Amounts if ASC 605 Had Been in Effect
Total revenue	$ 29,035	$ 33,159
Operating expenses	24,238	26,102
Operating income	4,797	$ 7,057
ASU 2014-09 | ASC 605 Presentation
Comparison to Amounts if ASC 605 Had Been in Effect
Total revenue	28,229
Operating expenses	23,432
Operating income	4,797
ASU 2014-09 | Reclassification
Comparison to Amounts if ASC 605 Had Been in Effect
Total revenue	(806)
Operating expenses	$ (806)