Note 11 - Net Loss Per Share Attributable to Common Stockholders (Details Textual) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accretion of Redeemable Convertible Preferred Stock to Redemption Value		$ 6,747

Represents the accretion of redeemable convertible preferred stock to redemption.

No definition available.

Name:	prto_AccretionOfRedeemableConvertiblePreferredStockToRedemptionValue
Namespace Prefix:	prto_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration