Impact of Topic 606 Adoption on Financial Statements (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 02, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract liability, current portion			$ 49,500			$ 6,389					$ 6,389
Contract liability, net of current portion						35,139					35,139
Accumulated deficit		$ (566,329)				(712,442)				$ (566,329)	(712,442)
Collaboration revenue		0	0	$ 0	$ 146	1,597	$ 1,598	$ 1,597	$ 1,597	146	6,389	$ 6,861
Net loss		$ 194,538	$ (18,659)	$ (17,493)	$ (21,540)	(18,272)	(17,604)	(17,727)	(19,658)	(53,117)	(54,323)	(165,297)
Net income (loss) applicable to common shareholders										$ 137,246	$ (73,261)	$ (172,420)
Net income (loss) per share—basic		$ 1.29	$ (0.13)	$ (0.12)	$ (0.15)					$ 0.96	$ (0.53)	$ (1.32)
Net income (loss) per share—diluted		$ 1.29	$ (0.13)	$ (0.12)	$ (0.15)					$ 0.96	$ (0.53)	$ (1.32)
Net loss		$ 194,538	$ (18,659)	$ (17,493)	$ (21,540)	$ (18,272)	$ (17,604)	$ (17,727)	$ (19,658)	$ (53,117)	$ (54,323)	$ (165,297)
Changes in contract liability										(146)	$ (6,389)	$ (6,861)
ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract liability, current portion													$ 622
Contract liability, net of current portion													49,037
Accumulated deficit													(720,573)
Changes in contract liability										0
Difference between Revenue Guidance in Effect before and after Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accumulated deficit													8,100
Changes in contract liability	$ 8,100
Difference between Revenue Guidance in Effect before and after Topic 606 | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract liability, current portion													(5,767)
Contract liability, net of current portion													13,898
Accumulated deficit													$ (8,131)
Collaboration revenue										(4,732)
Net loss										(4,732)
Net income (loss) applicable to common shareholders										$ (4,732)
Net income (loss) per share—basic										$ (0.03)
Net income (loss) per share—diluted										$ (0.03)
Net loss										$ (4,732)
Changes in contract liability										0
Calculated under Revenue Guidance in Effect before Topic 606 | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Collaboration revenue										4,878
Net loss										(48,385)
Net income (loss) applicable to common shareholders										$ 141,978
Net income (loss) per share—basic										$ 0.99
Net income (loss) per share—diluted										$ 0.99
Net loss										$ (48,385)
Changes in contract liability										$ 0