Purchase Commitments - Additional Information (Detail) - USD ($)		1 Months Ended		6 Months Ended
		Apr. 30, 2016	Jul. 31, 2014	Jun. 30, 2016	Dec. 31, 2015	Nov. 01, 2014
Long Term Purchase Commitment [Line Items]
Prepaid and other long-term assets	[1]			$ 16,018,000	$ 11,993,000
Prepaid research and development	[1]			22,478,000	16,976,000
Accrued research and development	[1]			$ 20,507,000	$ 24,195,000
CMC ICOS Biologics Inc
Long Term Purchase Commitment [Line Items]
Commercial supply agreement upfront payment			$ 10,000,000
Commercial supply (manufacturing services) agreement reservation payment						$ 4,600,000
Commercial agreement fixed commitment amount			276,100,000
Term of commercial supply agreement				7 years
Commercial supply agreement upfront and reservation payment				$ 14,600,000
Purchases of batches recorded in prepaid research and development				8,300,000
Accrued research and development				552,000
CMC ICOS Biologics Inc | Upfront and Reservation Payment
Long Term Purchase Commitment [Line Items]
Prepaid and other long-term assets				9,600,000
Prepaid research and development				$ 3,300,000
CMC ICOS Biologics Inc | Minimum
Long Term Purchase Commitment [Line Items]
Termination fees payment under obligation			5,000,000
CMC ICOS Biologics Inc | Maximum
Long Term Purchase Commitment [Line Items]
Termination fees payment under obligation			$ 30,000,000
Hovione, Limited
Long Term Purchase Commitment [Line Items]
Prepaid and other long-term assets		$ 6,000,000
Prepaid research and development		10,200,000
Commercial agreement advance payment amount		16,200,000
Hovione, Limited | Maximum
Long Term Purchase Commitment [Line Items]
Cancellable additional purchase commitments		$ 24,000,000

[1] Amounts include the assets and liabilities of our Development Partner 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 7, “Asset Acquisition and License Agreements,” to these condensed consolidated financial statements.