Net Loss per Share Attributable to Portola Common Stockholders - Outstanding Shares of Common Stock Shares Excluded from Computation of Diluted Net Loss per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Stock options to purchase Common Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of Earnings per share (in shares)	7,548,436	7,507,690	6,514,538
Performance stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of Earnings per share (in shares)	552,010	143,335	164,783
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of Earnings per share (in shares)	1,274,217	979,278	600,334
Performance stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of Earnings per share (in shares)	11,925	153,503	304,754
Common stock warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of Earnings per share (in shares)	1,500	1,500	1,500
Employee stock purchase plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of Earnings per share (in shares)	76,036	96,219	32,325