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Subsequent Events (Details) $ in Millions	6 Months Ended
	Jun. 30, 2016 USD ($)
Subsequent Event [Line Items]
One-time severance charge - Q3 2016 impact	$ 1.1

X

- Definition

One-time severance charge - Q3 2016 impact

+ References

No definition available.

+ Details

Name:	pro_OnetimeseverancechargeQ32016impact
Namespace Prefix:	pro_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

Name:	us-gaap_SubsequentEventLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration