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Segment Information (Narrative) (Details) | 12 Months Ended
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Dec. 31, 2015 Segment
Segment Reporting Information [Line Items] |
Number of operating segments | 4
Workers Compensation [Member] |
Segment Reporting Information [Line Items] |
Alternative market solutions percentage ceded | 100.00%
Lloyds Syndicate [Member] |
Segment Reporting Information [Line Items] |
Proportion of capital provided to support lloyd's syndicate | 58.00%

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- Definition Alternative Market Solutions Percentage Ceded + References No definition available. + Details
Name: | pra_AlternativeMarketSolutionsPercentageCeded
Namespace Prefix: | pra_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition This item represents the capital provided in support of a segment by a corporate member as a percentage of total capital provided by all members to support the segment. + References No definition available. + Details
Name: | pra_ProportionOfCapitalProvidedToSupportSegment
Namespace Prefix: | pra_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References No definition available. + Details
Name: | us-gaap_NumberOfOperatingSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- 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_SegmentReportingInformationLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=pra_WorkersCompensationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=pra_LloydsSyndicateMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |