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Employment Retirement Savings (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Contributions by Employer | $ 1.4us-gaap_DefinedBenefitPlanContributionsByEmployer | $ 1.1us-gaap_DefinedBenefitPlanContributionsByEmployer | $ 1.0us-gaap_DefinedBenefitPlanContributionsByEmployer
Matching Percentage of Salary Contribution by Qualified Employees | 50.00%pro_MatchingPercentageOfSalaryContributionByQualifiedEmployees |  |
Qualified Employees Contribution Matching Percentage by the Employer | 6.00%pro_QualifiedEmployeesContributionMatchingPercentageByEmployer |  |

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- Definition Matching percentage of salary contribution by qualified employees. + References No definition available. + Details
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- Definition Qualified employees contribution matching percentage by the employer. + References No definition available. + Details
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- Definition The increase in the fair value of plan assets from contributions made by the employer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
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- Details
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