Comparison of Reported Consolidated Statement of Operations Under New Guidance (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues:
Collaboration and license revenue									$ 40,130	$ 22,546	$ 35,504
Operating expenses:
Research and development									216,205	203,701	246,854
Loss from operations									(345,320)	(272,679)	(269,585)
Net loss	$ (88,886)	$ (71,177)	$ (105,971)	$ (84,510)	$ (91,501)	$ (82,941)	$ (69,414)	$ (41,764)	(350,544)	(285,620)	(268,113)
Net loss attributable to Portola	$ (88,548)	$ (71,303)	$ (106,194)	$ (84,178)	$ (91,781)	$ (82,936)	$ (69,654)	$ (41,719)	$ (350,223)	$ (286,090)	$ (269,043)
Basic and diluted	$ (1.34)	$ (1.08)	$ (1.61)	$ (1.28)	$ (1.41)	$ (1.41)	$ (1.22)	$ (0.74)	$ (5.31)	$ (4.81)	$ (4.76)
Collaboration and License Revenue
Revenues:
Collaboration and license revenue	$ 1,228	$ 7,001	$ 1,746	$ 6,038	$ 9,803	$ 3,828	$ 3,787	$ 5,128	$ 16,013	$ 22,546	$ 35,504
Balances Without the Adoption of ASC 606 | Accounting Standards Update 2014-09
Operating expenses:
Research and development									213,657
Loss from operations									(347,103)
Net loss									(352,327)
Net loss attributable to Portola									$ (352,006)
Basic and diluted									$ (5.33)
Balances Without the Adoption of ASC 606 | Collaboration and License Revenue | Accounting Standards Update 2014-09
Revenues:
Collaboration and license revenue									$ 11,682
Effect of Change | Accounting Standards Update 2014-09
Operating expenses:
Research and development									2,548
Loss from operations									1,783
Net loss									1,783
Net loss attributable to Portola									$ 1,783
Basic and diluted									$ 0.03
Effect of Change | Collaboration and License Revenue | Accounting Standards Update 2014-09
Revenues:
Collaboration and license revenue									$ 4,331