Revenue Recognition - Narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue	$ 700
Deferred expense	300					$ 400
Decrease to retained earnings	$ (84,728)	$ (83,421)	$ (77,338)	$ (75,554)	$ (72,360)		$ (71,453)
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred revenue						700
Decrease to retained earnings						$ 300