Net Loss per Share Attributable to Portola Common Stockholders - Outstanding Shares of Common Stock Equivalents Excluded from Computation of Diluted Net Loss per Share (Detail) - shares	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Stock options to purchase common stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	7,652,393	7,200,427
Performance stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	578,467	147,335
Common stock warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	1,500	1,500
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	1,253,581	809,014
Performance stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	13,125	160,378
Employee stock purchase plan
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from computation of diluted net loss per share	18,754	19,062