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Share-Based Transactions - Narrative (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 2,100,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Remaining Contractual Term   10 years    
Stock options exercise term, description Upon retirement, employees have the lesser of three years or the remaining option term to exercise any vested options.      
Equity compensation expense for the modified awards $ 4,600,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAcceleratedCompensationCost      
Share-based Compensation Arrangement by Share-based Payment Award, Sale Restrictions Expiration Period Subsequent to Vesting 3 years      
Restricted stock units (RSUs) [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized 7,100,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
     
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition 9 months 18 days      
Stock Options [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period   3 years    
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized 500,000us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Employee Service Share-based Compensation, Nonvested Awards, Total Compensation Cost Not yet Recognized, Period for Recognition 10 months 24 days      
Intrinsic value of options exercised 53,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Tax benefit realized from options exercised 19,000us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Value of issued shares that were withheld to satisfy the exercise price of stock options exercised $ 142,000pri_ValueOfSharesWithheldToSettleStrikePriceOfStockOptionsExercised
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
     
Nonemployee Shares [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Agent equity awards unvested 73,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= pri_NonemployeeSharesMember
104,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= pri_NonemployeeSharesMember
132,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= pri_NonemployeeSharesMember
117,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
/ us-gaap_AwardTypeAxis
= pri_NonemployeeSharesMember
Management | Restricted stock units (RSUs) [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 3 years      
Management | U.S. Based Subsidiaries [Member] | Restricted Stock [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period   3 years    
Management | Canadian Subsidiaries [Member] | Restricted stock units (RSUs) [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period   3 years    
Non-employee Directors [Member] | Restricted Stock [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period     3 years  
Non-employee Directors [Member] | Restricted stock units (RSUs) [Member]        
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period 1 year 1 year