v2.4.0.8

Variable Interest Entities (Details) (Variable Interest Entity, Not Primary Beneficiary [Member], USD $) In Millions, unless otherwise specified | Jun. 30, 2013 | Dec. 31, 2012
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Other Investment [Member] |  |
Variable Interest Entities (Textual) [Abstract] |  |
VIE interests carried as part of other investments | $ 28.4 | $ 25.1
VIE interest maximum loss exposure | 28.4 |
Equity Method Investments [Member] |  |
Variable Interest Entities (Textual) [Abstract] |  |
VIE interests carried as part of other investments | 27.3 | 33.7
VIE interest maximum loss exposure | $ 27.3 |

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- Details
Name: | pra_VariableInterestEntitiesTextualAbstract
Namespace Prefix: | pra_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph b -URI http://asc.fasb.org/extlink&oid=7880789&loc=d3e5728-111685 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=7880789&loc=d3e5747-111685 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name FASB Staff Position (FSP) -Number FAS140-4/FIN46(R) -Paragraph C6 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Carrying amount of the assets in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph a -URI http://asc.fasb.org/extlink&oid=7880789&loc=d3e5728-111685 + Details
Name: | us-gaap_VariableInterestEntityNonconsolidatedCarryingAmountAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant