Revenue Recognition - Summary of Impact of Adoption of New Revenue Standard on Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	$ 37,033		$ 31,699
Deferred cost	5,842		2,429
Deferred cost (non-current)	8,747
Other current liabilities	1,242		808
Deferred revenue (current and non-current)	88,639		73,072
Accumulated deficit	(137,865)		$ (109,252)
Accounting Standards Update 2014-09
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net		$ 31,799
Deferred cost		4,561
Deferred cost (non-current)		6,965
Other current liabilities		908
Deferred revenue (current and non-current)		73,072
Accumulated deficit		(100,155)
Accounting Standards Update 2014-09 | Adjustments Due to ASC 606
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	140	100
Deferred cost	1,481	2,132
Deferred cost (non-current)	8,747	6,965
Other current liabilities	140	100
Accumulated deficit	10,228	$ 9,097
Accounting Standards Update 2014-09 | Balances Without Adoption of ASC 606
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	36,893
Deferred cost	4,361
Other current liabilities	1,102
Deferred revenue (current and non-current)	88,639
Accumulated deficit	$ (148,093)