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Employment Retirement Savings (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	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

X

- Definition

Matching percentage of salary contribution by qualified employees.

+ References

No definition available.

+ Details

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X

- Definition

Qualified employees contribution matching percentage by the employer.

+ References

No definition available.

+ Details

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Namespace Prefix:	pro_
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X

- 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

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