Stock-Based Compensation - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015
Deferred Compensation Arrangement With Individual Share Based Payments [Line Items]
Percentage increase in number of common stock outstanding	5.00%
Number of common stock shares reserved for issuance under the 2013 Equity Incentive Plan	9,415,404
Restricted Stock Units (RSUs)
Deferred Compensation Arrangement With Individual Share Based Payments [Line Items]
Vesting period	3 years
Performance Shares
Deferred Compensation Arrangement With Individual Share Based Payments [Line Items]
Vesting period	4 years
Number of common stock earned per PSU	1
Minimum average market closing price to earn PSU	50
Consecutive trading period required to earn PSU	45 days
Minimum average closing price to earn PSU	60.00
Vesting rights	At any time during the four years following the date of the grant, half of the PSUs will vest one year after the date the average closing price of Portola’s stock on the NASDAQ Global Select Market is above $50.00 per share for 45 consecutive trading days, and an additional half of the shares will vest one year after the date the average closing price of Portola’s stock is above $60.00 per share for 45 consecutive trading days