Basis of presentation, principles of consolidation and significant accounting policies - Impact of Topic 606 Adoption on Condensed Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Basis Of Presentation And Significant Accounting Policies [Line Items]
Deferred revenue, current portion	$ 22,575	$ 45,344	$ 25,674
Deferred revenue, net of current portion	22,256	31,468	21,763
Accumulated deficit	(1,349,785)	(943,183)	$ (913,808)
ASU 2014-09 [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Deferred revenue, current portion	9,596	19,670
Deferred revenue, net of current portion	6,074	9,705
Accumulated deficit	15,670	(29,375)
ASU 2014-09 [Member] | Balances Without Adoption of Topic 606 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Deferred revenue, current portion	12,979	25,674
Deferred revenue, net of current portion	16,182	21,763
Accumulated deficit	$ (1,365,455)	$ (913,808)