Segment Information and Information about Geographic Area (Details) $ in Thousands	3 Months Ended						9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of operating segments (in segment) | segment							1
Revenues	$ 71,658	$ 68,153	$ 64,878	$ 59,490	$ 55,302	$ 53,121	$ 278,889	$ 230,828	$ 278,889	$ 230,828	$ 197,925	$ 197,925
Property and equipment, net	61,442						61,442		$ 61,442	58,557
United States
Segment Reporting Information [Line Items]
Revenues							185,887	162,681				139,743
Property and equipment, net	59,222						59,222			50,785
Foreign
Segment Reporting Information [Line Items]
Revenues							93,002	$ 68,147				$ 58,182
Property and equipment, net	$ 2,220						$ 2,220			$ 7,772