Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Operating activities
Net loss	$ (6,552)	$ (4,580)
Reconciliation of net loss to net cash used in operating activities:
Depreciation	24	8
Amortization of premium/discount on available-for-sale securities	36	$ 198
Unrealized gain on forward foreign currency contracts included in net income	(178)
Foreign currency remeasurement gain	(28)
Stock-based compensation	866	$ 407
Changes in:
Prepaid expenses and other assets	331	103
Interest receivable	(26)	(305)
Accounts payable, accrued expenses and other current liabilities	(367)	296
Net cash used in operating activities	(5,894)	(3,873)
Investing activities
Purchases of available-for-sale investments	(23,721)	(58,139)
Proceeds from maturities of available-for-sale investments	13,614	2,961
Purchase of property and equipment	(8)	(5)
Net cash used in investing activities	(10,115)	$ (55,183)
Financing activities
Exercise of stock options	12
Net cash provided by financing activities	12
Effect of exchange rate changes on cash	28
Decrease in cash and cash equivalents	(15,969)	$ (59,056)
Cash and cash equivalents, beginning of period	40,031	68,840
Cash and cash equivalents, end of period	$ 24,062	$ 9,784

Amount of gain (loss) from foreign currency remeasurement.

No definition available.

Name:	prto_ForeignCurrencyRemeasurementGainLoss
Namespace Prefix:	prto_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The sum of the periodic adjustments of the differences between securities' face values and purchase prices that are charged against earnings. This is called accretion if the security was purchased at a discount and amortization if it was purchased at premium. As a noncash item, this element is an adjustment to net income when calculating cash provided by or used in operations using the indirect method.