Investments - Gross Unrealized Gains and Losses on AFS Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Gross unrealized gains and losses on fixed maturity securities included in assets
Fixed maturity securities, available-for-sale, amortized cost	$ 2,038,469	$ 1,999,403
Credit Loss Allowance	(400)
Fair Value	2,002,420	2,040,682
Short-term Investments	19,229	43,873
Commercial levered loans (amortized cost)	13,725	14,069
Limited partnerships and limited liability companies (fair value)	65,011	66,660
Total investments	2,111,763	2,165,284
Fixed maturity securities
Gross unrealized gains and losses on fixed maturity securities included in assets
Fixed maturity securities, available-for-sale, amortized cost	2,038,469	1,999,403
Credit Loss Allowance	(416)
Gross Unrealized Gains	34,992	48,403
Gross Unrealized Losses	(70,625)	(7,124)
Fair Value	2,002,420	2,040,682
Fixed maturity securities \| U.S. Treasury securities
Gross unrealized gains and losses on fixed maturity securities included in assets
Fixed maturity securities, available-for-sale, amortized cost	40,679	49,161
Gross Unrealized Gains	2,643	838
Gross Unrealized Losses		(14)
Fair Value	43,322	49,985
Fixed maturity securities \| Government agency securities
Gross unrealized gains and losses on fixed maturity securities included in assets
Fixed maturity securities, available-for-sale, amortized cost	27,998	6,522
Gross Unrealized Gains	591	23
Gross Unrealized Losses		(14)
Fair Value	28,589	6,531
Fixed maturity securities \| Corporate debt securities
Gross unrealized gains and losses on fixed maturity securities included in assets
Fixed maturity securities, available-for-sale, amortized cost	1,317,091	1,308,094
Credit Loss Allowance	(293)
Gross Unrealized Gains	22,167	33,743
Gross Unrealized Losses	(37,278)	(3,025)
Fair Value	1,301,687	1,338,812
Fixed maturity securities \| Municipal debt obligations
Gross unrealized gains and losses on fixed maturity securities included in assets
Fixed maturity securities, available-for-sale, amortized cost	103,271	80,338
Gross Unrealized Gains	813	243
Gross Unrealized Losses	(3,443)	(766)
Fair Value	100,641	79,815
Fixed maturity securities \| ABS
Gross unrealized gains and losses on fixed maturity securities included in assets
Fixed maturity securities, available-for-sale, amortized cost	49,205	73,068
Credit Loss Allowance	(1)
Gross Unrealized Gains	26	854
Gross Unrealized Losses	(3,210)	(340)
Fair Value	46,020	73,582
Fixed maturity securities \| CLO
Gross unrealized gains and losses on fixed maturity securities included in assets
Fixed maturity securities, available-for-sale, amortized cost	177,411	181,704
Credit Loss Allowance	(6)
Gross Unrealized Gains		125
Gross Unrealized Losses	(19,842)	(2,280)
Fair Value	157,563	179,549
Fixed maturity securities \| CMBS
Gross unrealized gains and losses on fixed maturity securities included in assets
Fixed maturity securities, available-for-sale, amortized cost	94,816	95,810
Gross Unrealized Gains	1,603	1,863
Gross Unrealized Losses	(1,945)	(147)
Fair Value	94,474	97,526
Fixed maturity securities \| RMBS - non-agency
Gross unrealized gains and losses on fixed maturity securities included in assets
Fixed maturity securities, available-for-sale, amortized cost	92,189	62,343
Credit Loss Allowance	(116)
Gross Unrealized Gains	3,411	9,458
Gross Unrealized Losses	(4,851)	(191)
Fair Value	90,633	71,610
Fixed maturity securities \| RMBS - agency
Gross unrealized gains and losses on fixed maturity securities included in assets
Fixed maturity securities, available-for-sale, amortized cost	135,809	142,363
Gross Unrealized Gains	3,738	1,256
Gross Unrealized Losses	(56)	(347)
Fair Value	$ 139,491	$ 143,272

Represents the amortized cost of commercial levered loans.

No definition available.

Name:	pros_CommercialLeveredLoansAmortizedCost
Namespace Prefix:	pros_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the fair value of investments in limited partnerships and limited liability companies.

+ References

No definition available.

+ Details

Name:	pros_LimitedPartnershipsAndLimitedLiabilityCompaniesFairValue
Namespace Prefix:	pros_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- Definition

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

+ Details

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

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

+ Details

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

- References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesFairValueToAmortizedCostBasisAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922900-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 9
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922900-210455

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aaa)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Sum of the carrying amounts as of the balance sheet date of all investments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(h))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

+ Details

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

- Definition

Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(4))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(6))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(5))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(1)(g))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_FixedIncomeInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MunicipalNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedDebtObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=pros_ResidentialMortgageBackedSecuritiesNonAgencyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=pros_ResidentialMortgageBackedSecuritiesAgencyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: