Selected quarterly financial data - Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 1,552	$ 1,552	$ 1,552	$ 1,499	$ 1,471	$ 1,324	$ 4,940	$ 6,344	$ 6,155	$ 14,079	$ 25,421
Total operating expenses	73,887	79,692	61,527	58,879	50,432	44,451	56,963	31,270	273,985	183,116	86,047
Loss from operations	(72,335)	(78,140)	(59,975)	(57,380)	(48,961)	(43,127)	(52,023)	(24,926)	(267,830)	(169,037)	(60,626)
Net loss	$ (71,364)	$ (77,025)	$ (58,844)	$ (56,274)	$ (47,277)	$ (42,924)	$ (51,795)	$ (24,787)	$ (263,507)	$ (166,783)	$ (48,709)
Net loss per share applicable to common stockholders - basic and diluted	$ (1.88)	$ (2.07)	$ (1.59)	$ (1.52)	$ (1.29)	$ (1.18)	$ (1.57)	$ (0.76)	$ (7.07)	$ (4.81)	$ (1.83)