Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015		Dec. 31, 2014	Dec. 31, 2013
Summary Of Significant Accounting Policies [Line Items]
Prepaid expenses and other current assets	$ 3,059,000	[1]	$ 4,061,000
Prepaid research and development	16,976,000	[1]	1,686,000
Accrued and other liabilities	2,826,000	[1]	1,421,000
Accrued research and development	24,195,000	[1]	12,545,000
Impairment losses	0
Interest or penalties charged to the underpayment of income taxes	0
Foreign currency forward Contract	$ 0		0
Unrealized loss			(114,000)	$ 261,000
Realized loss			(258,000)
Minimum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment useful lives	2 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment useful lives	5 years
Prior-Period Reclassified Amount
Summary Of Significant Accounting Policies [Line Items]
Prepaid expenses and other current assets			5,700,000
Accrued and other liabilities			$ 14,000,000

[1] Amounts include the assets and liabilities of a consolidated variable interest entity (“VIE”). Portola's interests and obligations with respect to the VIE's assets and liabilities are limited to those accorded to Portola in its agreement with the VIE. See Note 8, “Asset Acquisition and License Agreements,” to these consolidated financial statements.