Asset Acquisition and License Agreements - Additional Information (Detail) - USD ($)		1 Months Ended			12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Nov. 30, 2007	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2011	Dec. 01, 2015
Asset Acquisition [Line Items]
Restricted cash (Development Partner)	[1]	$ 341,000			$ 341,000
Research and development					$ 200,376,000	$ 123,639,000	$ 79,286,000
Millennium
Asset Acquisition [Line Items]
License fees paid				$ 5,000,000
Research and development				$ 5,000,000
Millennium | Maximum
Asset Acquisition [Line Items]
Additional license fees payable on occurrence of specified events			$ 35,000,000
Astellas Pharma Inc
Asset Acquisition [Line Items]
License fees paid								$ 7,200,000
Research and development								$ 7,200,000
Percentage required to pay for any payment received under collaboration excluding royalties			20.00%			20.00%
Astellas Pharma Inc | Maximum
Asset Acquisition [Line Items]
Additional license fees payable on occurrence of specified events			$ 71,500,000
Development Partner
Asset Acquisition [Line Items]
Variable interest entity, Terms of arrangements					Accordingly, the Development Partner is subject to consolidation and we have consolidated the financial statements of the Development Partner since inception of the agreement on December 1, 2015 by (a) eliminating all intercompany balances and transactions; (b) allocating loss attributable to the noncontrolling interest in the Development Partner to net loss attributable to noncontrolling interest in our consolidated statement of operations and reflecting noncontrolling interest on our consolidated balance sheet.
Variable interest entity, Upfront payment		500,000
Intellectual property asset at fair value									$ 3,200,000
Noncontrolling interest in variable Interest entity		2,900,000			$ 2,900,000				$ 2,900,000
Research and development expense, In process		3,200,000
Restricted cash (Development Partner)		$ 341,000			341,000
Net loss attributable to noncontrolling interest (Development Partner)					$ 0

[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.