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Commitments and Contingent Liabilities - Narrative (Details) $ in Millions | 6 Months Ended
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Jun. 30, 2018 USD ($) Jurisdiction
Commitments And Contingent Liabilities [Line Items] |
Number of jurisdiction | Jurisdiction | 30
Letter of Credit [Member] |
Commitments And Contingent Liabilities [Line Items] |
Letter of credit, outstanding | $ | $ 326.2

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- Definition Commitments and contingent liabilities. + References No definition available. + Details
Name: | pri_CommitmentsAndContingentLiabilitiesLineItems
Namespace Prefix: | pri_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of jurisdiction. + References No definition available. + Details
Name: | pri_NumberOfJurisdiction
Namespace Prefix: | pri_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
Name: | us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_CreditFacilityAxis=us-gaap_LetterOfCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |