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Condensed Consolidated Statements of Shareholders' Equity (Parentheticals) (USD $)
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Shareholders Equity Parentheticals    
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest $ 7,254,828,000us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest $ 6,840,599,000us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest
Unfunded pension obligation [Member]    
Shareholders Equity Parentheticals    
Balance at end of period, unfunded pension obligation, tax impact 8,432,000pre_OtherAccumulatedComprehensiveIncomeLossTax
/ us-gaap_OtherComprehensiveIncomeLocationAxis
= pre_UnfundedPensionObligationMember		 5,023,000pre_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