Purchase Commitments - Additional Information (Detail)		1 Months Ended		9 Months Ended
		Apr. 30, 2016 USD ($)	Jul. 31, 2014 USD ($)	Sep. 30, 2016 USD ($) ManufacturingLine l	Dec. 31, 2015 USD ($)	Nov. 01, 2014 USD ($)
Long Term Purchase Commitment [Line Items]
Prepaid and other long-term assets	[1]			$ 6,316,000	$ 11,993,000
Prepaid research and development	[1]			15,575,000	16,976,000
Prepaid expenses and other current assets	[1]			4,596,000	3,059,000
Accounts payables	[1]			8,993,000	10,279,000
Accrued research and development	[1]			$ 15,433,000	$ 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
Number of manufacturing lines | ManufacturingLine				2
Commercial agreement fixed commitment amount			276,100,000
Term of commercial supply agreement				7 years
Commercial supply agreement unamortized upfront and reservation payment				$ 12,300,000
Prepaid research and development				352,000
Purchases of batches recorded in prepaid research and development				13,300,000
Prepaid expenses and other current assets				1,300,000
Accounts payables				367,000
Accrued research and development				1,800,000
CMC ICOS Biologics Inc | Other Miscellaneous Receivables
Long Term Purchase Commitment [Line Items]
Prepaid expenses and other current assets				1,700,000
CMC ICOS Biologics Inc | Research and Development Expense
Long Term Purchase Commitment [Line Items]
Charge				27,300,000
CMC ICOS Biologics Inc | Unamortized Upfront and Reservation Payment
Long Term Purchase Commitment [Line Items]
Prepaid and other long-term assets				8,900,000
Prepaid research and development				$ 3,400,000
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
CMC ICOS Biologics Inc | Line A/B
Long Term Purchase Commitment [Line Items]
Purchase commitment, description				(i) the 2,500 liter manufacturing line which has been used since inception of the program to supply clinical drug product, referred to as “Line A/B”
Purchase commitment volume | l				2,500
CMC ICOS Biologics Inc | Line C
Long Term Purchase Commitment [Line Items]
Purchase commitment, description				(ii) the 6x2,000 liter manufacturing line referred to as “Line C” which was intended to satisfy the drug product requirements of our initial commercial launch.
Purchase commitment volume | l				12,000
Hovione, Limited
Long Term Purchase Commitment [Line Items]
Prepaid and other long-term assets		$ 6,000,000
Prepaid research and development		12,700,000
Commercial agreement advance payment amount		18,700,000
Hovione, Limited | Maximum
Long Term Purchase Commitment [Line Items]
Cancellable additional purchase commitments		$ 24,100,000

[1] Amounts include the assets and liabilities of SRX Cardio, LLC, 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.