Commercial Supply Agreement - Additional Information (Detail) - USD ($)		1 Months Ended	3 Months Ended
		Jul. 31, 2014	Mar. 31, 2016	Dec. 31, 2015	Nov. 01, 2014
Long Term Purchase Commitment [Line Items]
Prepaid and other long-term assets	[1]		$ 11,177,000	$ 11,993,000
Prepaid research and development	[1]		10,854,000	16,976,000
Accrued research and development	[1]		17,574,000	$ 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
Payment made for purchases of batches related to commercial supply agreement since inception			$ 40,700,000
Term of commercial supply agreement			7 years
Purchases of batches recorded in prepaid research and development			$ 7,700,000
Accrued research and development			857,000
CMC ICOS Biologics Inc | Upfront and Reservation Payment
Long Term Purchase Commitment [Line Items]
Prepaid expenses and other assets			14,600,000
Prepaid and other long-term assets			10,600,000
Prepaid research and development			$ 2,800,000
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

[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.