Revenues - Narrative (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jul. 01, 2018	Dec. 31, 2017
Revenue from Contract with Customer [Abstract]
Contract with customer, deferred revenues	$ 107,413	$ 113,300	$ 96,383	$ 104,400
Contract with customer, deferred revenues, current	94,200
Contract with customer, deferred revenues, noncurrent	13,200
Deferred sales commission	$ 3,300		$ 2,300