Financial Instruments - Additional Information (Detail)	9 Months Ended
Sep. 30, 2018
Investments Debt And Equity Securities [Abstract]
Available-for-sale securities, contractual maturities	2 years

Available for sale securities maximum remaining contractual maturity.

No definition available.

Name:	ptla_AvailableForSaleSecuritiesMaximumRemainingContractualMaturity
Namespace Prefix:	ptla_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration