Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	Mar. 31, 2017 USD ($)
ASU No. 2016-09
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect of differed tax assets	$ 14.0

New accounting pronouncement or change in accounting principle cumulative effect of differed tax assets.

No definition available.

Name:	ptla_NewAccountingPronouncementOrChangeInAccountingPrincipleCumulativeEffectOfDifferedTaxAssets
Namespace Prefix:	ptla_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant