Condensed Quarterly Financial Data (unaudited) - Schedule of Condensed Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 30,032	$ 28,616	$ 24,999	$ 23,085	$ 35,234	$ 27,016	$ 34,592	$ 18,063	$ 106,732	$ 114,905	$ 86,489
Product and service gross profit	12,848	12,162	11,832	10,350	11,832	9,610	10,086	8,602
Net loss	$ (21,111)	$ (16,486)	$ (20,601)	$ (19,202)	$ (8,751)	$ (11,404)	$ (4,555)	$ (18,852)	$ (77,400)	$ (43,562)	$ (47,089)
Net loss per share - basic and diluted (in dollars per share)	$ (0.68)	$ (0.56)	$ (0.80)	$ (0.75)	$ (0.34)	$ (0.45)	$ (0.20)	$ (0.87)	$ (2.78)	$ (1.84)	$ (2.34)