Summary of Significant Accounting Policies Significant Accounting Policies (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Dec. 31, 2018	Jan. 01, 2018
Summary of Significant Accounting Policies [Line Items]
Tangible Asset Impairment Charges	$ 0
Total shareholder return period, in years, for vesting of MSUs	3 years
Shares issuable upon vesting of MSUs, maximum	200.00%
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized	$ 33,266
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition	2 years 4 months 13 days
Share-based Compensation Award, Tranche One [Member]
Summary of Significant Accounting Policies [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Awards vesting upon Price Target	25.00%
Share Price Target	$ 27
Share-based Compensation Award, Tranche Two [Member]
Summary of Significant Accounting Policies [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Awards vesting upon Price Target	25.00%
Share Price Target	$ 33
Share-based Compensation Award, Tranche Three [Member]
Summary of Significant Accounting Policies [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Awards vesting upon Price Target	50.00%
Share Price Target	$ 41
Accounting Standards Update 2014-09 [Member]
Summary of Significant Accounting Policies [Line Items]
New Accounting Pronouncement, Cumulative Effect of Change on Equity, Impact of Revenue		$ 2,700
New Accounting Pronouncement, Cumulative Effect of Change on Equity, Impact of Cost to Obtain Contract		$ 7,000

New Accounting Pronouncement, Cumulative Effect of Change on Equity, Impact of Cost to Obtain Contract

No definition available.

Name:	pro_NewAccountingPronouncementCumulativeEffectofChangeonEquityImpactofCosttoObtainContract
Namespace Prefix:	pro_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

New Accounting Pronouncement, Cumulative Effect of Change on Equity, Impact of Revenue

+ References

No definition available.

+ Details

Name:	pro_NewAccountingPronouncementCumulativeEffectofChangeonEquityImpactofRevenue
Namespace Prefix:	pro_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Share Price Target

+ References

No definition available.

+ Details

Name:	pro_SharePriceTarget
Namespace Prefix:	pro_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Awards vesting upon Price Target

+ References

No definition available.

+ Details

Name:	pro_SharebasedCompensationArrangementbySharebasedPaymentAwardPercentageofAwardsvestinguponPriceTarget
Namespace Prefix:	pro_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Maximum number of shares issuable upon vesting of MSUs initially granted.

+ References

No definition available.

+ Details

Name:	pro_SharesIssuableUponVestingOfMsusMaximum
Namespace Prefix:	pro_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Summary of Significant Accounting Policies [Line Items]

+ References

No definition available.

+ Details

Name:	pro_SummaryOfSignificantAccountingPoliciesLineItems
Namespace Prefix:	pro_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Total shareholder return period used to determine vesting of MSUs.

+ References

No definition available.

+ Details

Name:	pro_TotalshareholderreturnperiodforvestingofMSUs
Namespace Prefix:	pro_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Unrecognized cost of unvested share-based compensation awards.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The charge against earnings resulting from the aggregate write down of tangible assets from their carrying value to their fair value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84176398&loc=d3e2921-110230

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=64827811&loc=d3e1361-107760

+ Details

Name:	us-gaap_TangibleAssetImpairmentCharges
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	pro_MarketbasedRSUAwardTrancheAxis=us-gaap_ShareBasedCompensationAwardTrancheOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	pro_MarketbasedRSUAwardTrancheAxis=us-gaap_ShareBasedCompensationAwardTrancheTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	pro_MarketbasedRSUAwardTrancheAxis=us-gaap_ShareBasedCompensationAwardTrancheThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AdjustmentsForNewAccountingPronouncementsAxis=us-gaap_AccountingStandardsUpdate201409Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: