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Summary of Significant Accounting Policies Significant Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023
Accounting Policies [Abstract]
Treasury money market funds, at fair value	$ 150.6		$ 153.2
Deferred Costs	14.5		$ 15.1
Amortization of Deferred Charges	$ 1.3	$ 1.5

X

- Definition

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

+ References

No definition available.

+ Details

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

X

- References

No definition available.

+ Details

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X

- Definition

The amount of amortization of deferred charges applied against earnings during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 220
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.3)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483621/220-10-S99-2

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X

- Definition

The carrying amount of deferred costs.

+ References

No definition available.

+ Details

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