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Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value Disclosures [Abstract]
Treasury money market funds, at fair value	$ 135.3pro_TreasuryMoneyMarketFundsAtFairValue	$ 21.0pro_TreasuryMoneyMarketFundsAtFairValue

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- Definition

Investments in treasury money market funds which are part of the company's assets, measured at fair value.

+ References

No definition available.

+ Details

Name:	pro_TreasuryMoneyMarketFundsAtFairValue
Namespace Prefix:	pro_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- Details

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