Summary of Significant Accounting Principles - Components of Revenue from Contracts with Customers (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from contracts with customers:
Total revenue from contracts with customers									$ 270,607	$ 247,880	$ 186,744
Product sales recognized under ASC 840									5,533	6,340
Total revenues	$ 66,337	$ 71,042	$ 70,788	$ 67,973	$ 75,976	$ 69,366	$ 56,957	$ 51,921	276,140	254,220	186,744
Recurring Service Revenues [Member]
Revenue from contracts with customers:
Total revenue from contracts with customers									148,367	126,540	106,515
Other Service Revenues [Member]
Revenue from contracts with customers:
Total revenue from contracts with customers									5,222	8,398	6,366
Service [Member]
Revenue from contracts with customers:
Total revenue from contracts with customers									153,589	134,938	112,881
Product [Member]
Revenue from contracts with customers:
Total revenue from contracts with customers									117,018	112,942	73,863
Total revenues									$ 122,551	$ 119,282	$ 73,863