Quarterly Financial Information (unaudited) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Data [Abstract]
Collaboration and license revenue	$ 4,414	$ 2,912	$ 2,385	$ 2,359	$ 2,411	$ 2,427	$ 2,415	$ 2,372	$ 12,070	$ 9,625	$ 10,531
Operating expenses	(70,694)	(58,476)	(61,212)	(48,863)	(41,671)	(38,204)	(33,920)	(33,396)	(239,245)	(147,191)	(94,709)
Net loss	$ (66,105)	$ (55,158)	$ (58,329)	$ (46,913)	$ (39,256)	$ (35,793)	$ (31,350)	$ (30,726)	$ (226,505)	$ (137,125)	$ (83,352)
Net loss per share attributable to Portola common stockholders:
Basic and diluted	$ (1.23)	$ (1.05)	$ (1.12)	$ (0.95)	$ (0.82)	$ (0.86)	$ (0.76)	$ (0.75)	$ (4.36)	$ (3.19)	$ (3.65)