Cumulative Effect to Condensed Consolidated Balance Sheet for Adoption of New Revenue Standard (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current portion	$ 3,444		$ 41,705
Deferred revenue, net of current portion	1,623		43,075
Accumulated deficit	(2,629,531)		$ (2,147,685)
ASU 2014-09 Revenue from Contracts with Customers
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current portion		$ 7,242
Deferred revenue, net of current portion		9,328
Accumulated deficit		(2,079,475)
ASU 2014-09 Revenue from Contracts with Customers | Effect of Change Higher/(Lower)
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current portion		(34,463)
Deferred revenue, net of current portion	(17,992)	(33,747)
Accumulated deficit	$ (18,077)	$ 68,210