Summary of Significant Accounting Principles - Components of Revenue from Contracts with Customers (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue from contracts with customers:
Total revenue from contracts with customers	$ 69,609	$ 66,939	$ 204,957	$ 175,817
Product sales recognized under ASC 840	1,433	2,427	4,846	2,427
Total revenues	71,042	69,366	209,803	178,244
Recurring Service Revenues [Member]
Revenue from contracts with customers:
Total revenue from contracts with customers	37,192	32,992	111,010	91,003
Other Service Revenues [Member]
Revenue from contracts with customers:
Total revenue from contracts with customers	1,281	2,048	3,930	4,626
Service Revenues [Member]
Revenue from contracts with customers:
Total revenue from contracts with customers	38,473	35,040	114,940	95,629
Product [Member]
Revenue from contracts with customers:
Total revenue from contracts with customers	31,136	31,899	90,017	80,188
Total revenues	$ 32,569	$ 34,326	$ 94,863	$ 82,615