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Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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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