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Consolidated Statements of Shareholders' Equity (Parentheticals) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Unfunded pension obligation [Member]      
Shareholders Equity Parentheticals      
Balance at end of period, unfunded pension obligation, tax impact $ 8,301,000pre_OtherAccumulatedComprehensiveIncomeLossTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= pre_UnfundedPensionObligationMember
$ 5,029,000pre_OtherAccumulatedComprehensiveIncomeLossTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= pre_UnfundedPensionObligationMember
$ 7,731,000pre_OtherAccumulatedComprehensiveIncomeLossTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= pre_UnfundedPensionObligationMember
Unrealized gain on investments [Member]      
Shareholders Equity Parentheticals      
Balance at end of period, unrealized gain on investments, tax impact $ 0us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= pre_UnrealizedGainsLossesOnInvestmentsMember
$ 0us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= pre_UnrealizedGainsLossesOnInvestmentsMember
$ 0us-gaap_OtherComprehensiveIncomeUnrealizedGainLossOnDerivativesArisingDuringPeriodTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= pre_UnrealizedGainsLossesOnInvestmentsMember