Basis of presentation, principles of consolidation and significant accounting policies - Impact of Topic 606 Adoption on Condensed Consolidated Statement of Cash Flows (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basis Of Presentation And Significant Accounting Policies [Line Items]
Net loss	$ (145,480)	$ (78,805)	$ (406,602)	$ (218,415)
Changes in deferred revenue			(31,981)	$ 3,084
ASU 2014-09 [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Net loss	4,886		13,705
Changes in deferred revenue			(13,705)
ASU 2014-09 [Member] | Balances Without Adoption of Topic 606 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Net loss	$ (150,366)		(420,307)
Changes in deferred revenue			$ (18,276)