Available For Sale Securities In Unrealized Loss Positions.

No definition available.

Name:	pra_AvailableForSaleSecuritiesInUnrealizedLossPositionsNumberOfIssuers
Namespace Prefix:	pra_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Business Owned Life Insurance Cost.

+ References

No definition available.

+ Details

Name:	pra_BusinessOwnedLifeInsuranceCost
Namespace Prefix:	pra_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Debt Securities, Number of Issuers

+ References

No definition available.

+ Details

Name:	pra_DebtSecuritiesNumberofIssuers
Namespace Prefix:	pra_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The total amount of equity in earnings (loss) of unconsolidated subsidiaries that resulted from the conversion of an investment to the equity method of accounting.

+ References

No definition available.

+ Details

Name:	pra_EquityInEarningsLossofUnconsolidatedSubsidiariesEffectofConversiontoEquityMethodTotal
Namespace Prefix:	pra_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of equity in earnings (loss) of unconsolidated subsidiaries that resulted from the conversion of an investment to the equity method of accounting in the current year that would have been recognized in prior years had the investment been accounted for under the equity method.

+ References

No definition available.

+ Details

Name:	pra_EquityinEarningsLossofUnconsolidatedSubsidiariesEffectofConversiontoEquityMethodPriorYears
Namespace Prefix:	pra_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Number of debt securities in unrealized loss position as percentage of total debt securities.

+ References

No definition available.

+ Details

Name:	pra_NumberOfDebtSecuritiesInUnrealizedLossPositionAsPercentageOfDebtSecurities
Namespace Prefix:	pra_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of Investment Affiliates Exceeding Shareholder's Equity Ten Percent Threshold Limit

+ References

No definition available.

+ Details

Name:	pra_NumberOfInvestmentAffiliatesExceedingShareholdersEquityTenPercentThresholdLimit
Namespace Prefix:	pra_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of Tax Credit Partnerships Almost 100% Ownership Percentage

+ References

No definition available.

+ Details

Name:	pra_NumberOfTaxCreditPartnershipsAlmost100OwnershipPercentage
Namespace Prefix:	pra_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Period For Federal Home Loan Bank Stock Liquidation Process

+ References

No definition available.

+ Details

Name:	pra_PeriodForFederalHomeLoanBankStockLiquidationProcess
Namespace Prefix:	pra_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Second greatest unrealized loss position.

+ References

No definition available.

+ Details

Name:	pra_SecondGreatestUnrealizedLossPosition
Namespace Prefix:	pra_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Single greatest unrealized loss position.

+ References

No definition available.

+ Details

Name:	pra_SingleGreatestUnrealizedLossPosition
Namespace Prefix:	pra_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Threshold limit of investments based on shareholders equity.

+ References

No definition available.

+ Details

Name:	pra_ThresholdLimitOfInvestmentsBasedOnShareholdersEquity
Namespace Prefix:	pra_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of affordable housing tax credits and other tax benefits recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 740
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=49127445&loc=SL49122993-202985

+ Details

Name:	us-gaap_AffordableHousingTaxCreditsAndOtherTaxBenefitsAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value of investments in certain entities that calculate net asset value per share, including, but not limited to, by unit, membership interest, or other ownership interest. Investments include, but are not limited to, investments in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=SL6742756-110258

+ Details

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

- Definition

Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 320
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=28366721&loc=d3e22002-158495

+ Details

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

- Definition

The fair value of available-for-sale securities that serve as collateral for borrowings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5086-111524

+ Details

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

- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

+ Details

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

This item represents an other than temporary decline in value that has been recognized against an investment accounted for under the cost method of accounting. The excess of the carrying amount over the fair value of the investment represents the amount of the write down which is or was reflected in earnings. The written down value is a new cost basis with the adjusted value of the investment becoming its new carrying value. Evidence of a loss in value might include, but would not necessarily be limited to, absence of an ability to recover the carrying amount of the investment or inability of the investee to sustain an earnings capacity which would justify the carrying amount of the investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 325
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6872867&loc=d3e40691-111596

+ Details

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

- Definition

The carrying amount of the asset transferred to a third party to serve as a deposit, which typically serves as security against failure by the transferor to perform under terms of an agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8,17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571

+ Details

Name:	us-gaap_EquityMethodInvestmentOwnershipPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=35755396&loc=d3e33749-111570

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.12)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Amount of investment income (loss) of qualified affordable housing project investments accounted for using the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 740
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=49127445&loc=SL49122993-202985

+ Details

Name:	us-gaap_IncomeLossFromAffordableHousingProjectsEquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563

+ Details

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

- Definition

Amount before tax of other than temporary impairment (OTTI) loss on investments in available-for-sale and held-to-maturity debt securities, recognized in other comprehensive loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 35
-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25362-111560

+ Details

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

- Definition

The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-URI http://asc.fasb.org/extlink&oid=27724398&loc=SL6284393-111563

+ Details

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

- Definition

Amount of real estate investment property, net of accumulated depreciation, which may include the following: (1) land available-for-sale; (2) land available-for-development; (3) investments in building and building improvements; (4) tenant allowances; (5) developments in-process; (6) rental properties; and (7) other real estate investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(d))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=pra_TaxCreditPartnershipsAlmost100OwnershipMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=pra_TaxCreditPartnershipsLessThan20OwnershipMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=pra_OtherLimitedPartnershipsandLimitedLiabilityCompanyGreaterThan25PercentOwnershipMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis=pra_OtherLimitedPartnershipsandLimitedLiabilityCompanyLessthan25PercentOwnershipMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_ShortTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=pra_NonGovernmentBackedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Investments (Narrative) (Details) $ in Thousands	12 Months Ended
Investments (Narrative) (Details) $ in Thousands	Dec. 31, 2015 USD ($) Affiliate investment_interest Issuer Security	Dec. 31, 2014 USD ($) Issuer Security	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Number of investment affiliates exceeding shareholder's equity ten percent threshold limit \| Affiliate	0
Threshold limit of investments based on shareholders' equity	10.00%
Securities on deposit with state insurance departments	$ 48,800
Business owned life insurance cost	33,000
Investment in unconsolidated subsidiaries	311,908	$ 276,501
Investments in limited liability companies and limited partnerships fair value	162,624	133,250
Estimated fair value of interest	19,400	10,100
Equity in earnings (loss) from unconsolidated subsidiaries	10,100	10,700	$ 10,100
Tax credits related to qualified affordable housing investments	18,400	17,900	17,900
Effect of conversion to equity method, total			10,500
Effect of conversion to equity method, prior years			8,400
Net impairments recognized in earnings	15,345	1,207	71
Credit-related OTTI	5,751	$ 232	$ 83	$ 3,301
OTTI cost-method investment	$ 8,100
Tax Credit Partnerships Almost 100% Ownership [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of tax credit partnerships almost 100% ownership percentage \| investment_interest	2
Investment in unconsolidated subsidiaries	$ 53,400
Tax Credit Partnerships Less Than 20% Ownership [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment in unconsolidated subsidiaries	$ 68,100
Other Limited Partnerships and Limited Liability Company, Greater Than 25% Ownership [Member]
Schedule of Available-for-sale Securities [Line Items]
Percentage ownership (greater than)	25.00%
Investment in unconsolidated subsidiaries	$ 8,300
Other Limited Partnerships and Limited Liability Company Less than 25% Ownership [Member]
Schedule of Available-for-sale Securities [Line Items]
Percentage ownership (greater than)	25.00%
Investment in unconsolidated subsidiaries	$ 173,700
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Period for federal home loan bank stock liquidation process	5 years
Maximum [Member] \| Tax Credit Partnerships Almost 100% Ownership [Member]
Schedule of Available-for-sale Securities [Line Items]
Percentage ownership (greater than)	100.00%
Maximum [Member] \| Tax Credit Partnerships Less Than 20% Ownership [Member]
Schedule of Available-for-sale Securities [Line Items]
Percentage ownership (greater than)	20.00%
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
FAL Deposit assets	$ 95,400
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
FAL Deposit assets	$ 400
Non Government-Backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of debt securities in unrealized loss position \| Security	773	588
Debt securities in unrealized loss position as percentage of total debt securities held	28.80%	20.50%
Number of issuers in unrealized loss position \| Issuer	506	434
Single greatest unrealized loss position	$ 1,400	$ 1,700
Second greatest unrealized loss position	1,300	$ 700
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Net impairments recognized in earnings	7,200
Credit-related OTTI	$ 4,900
Debt Securities, Number of Issuers \| Issuer	6
Non-credit impairment	$ 3,700
Revolving Credit Facility [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities pledged as collateral	$ 125,600