Impact on Condensed Consolidated Balance Sheet and Consolidated Statement of Comprehensive Loss for Adoption of New Revenue Standard (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current portion	$ 3,444		$ 3,444			$ 41,705
Deferred revenue, net of current portion	1,623		1,623			43,075
Accumulated deficit	(2,629,531)		(2,629,531)			$ (2,147,685)
Revenues	2,069	$ 17,096	53,875	$ 51,988
Net loss	$ (245,282)	$ (122,937)	$ (550,056)	$ (348,647)
Net loss per common share - basic and diluted	$ (2.43)	$ (1.34)	$ (5.48)	$ (3.93)
Net Revenues from Collaborators
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 1,609	$ 17,096	$ 53,415	$ 51,988
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)		(17,992)		(33,747)
Accumulated deficit	(18,077)		(18,077)		$ 68,210
Net loss	698		$ 50,133
Net loss per common share - basic and diluted			$ 0.50
ASU 2014-09 Revenue from Contracts with Customers | Effect of Change Higher/(Lower) | Net Revenues from Collaborators
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	(698)		$ (50,133)
ASU 2014-09 Revenue from Contracts with Customers | Balances Without Adoption of New Revenue Standard
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Deferred revenue, current portion	3,444		3,444
Deferred revenue, net of current portion	19,615		19,615
Accumulated deficit	(2,647,608)		(2,647,608)
Net loss	$ (244,584)		$ (499,923)
Net loss per common share - basic and diluted	$ (2.43)		$ (4.98)
ASU 2014-09 Revenue from Contracts with Customers | Balances Without Adoption of New Revenue Standard | Net Revenues from Collaborators
Revenue Initial Application Period Cumulative Effect Transition [Line Items]
Revenues	$ 2,307		$ 103,548