Basis of Presentation and Summary of Significant Accounting Policies - Summary of Impacts of Adopting the New Standard on Balance Sheet for Adoption Date (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
ASSETS
Deferred costs, current	$ 5,765		$ 0
Other assets including non-current deferred costs	19,393		1,593
LIABILITIES
Deferred revenue, current	128,519		126,174
Deferred revenue	46,132		33,081
STOCKHOLDERS' EQUITY:
Accumulated deficit	(269,904)		$ (256,537)
Topic 606 [Member]
ASSETS
Deferred costs, current		$ 3,755
Other assets including non-current deferred costs		13,749
LIABILITIES
Deferred revenue, current		122,015
Deferred revenue		33,678
STOCKHOLDERS' EQUITY:
Accumulated deficit		(237,064)
Impact of Adoption [Member] | Topic 606 [Member]
ASSETS
Deferred costs, current	(5,765)	3,755
Other assets including non-current deferred costs	(17,941)	12,156
LIABILITIES
Deferred revenue, current	5,769	(4,159)
Deferred revenue	1,989	597
STOCKHOLDERS' EQUITY:
Accumulated deficit	$ (31,464)	$ 19,473