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Share-Based Awards (Details - Narrative)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Employee Share Purchase Plan [Member]  
Employee Share Purchase Plans [Line Items]  
Length of offering period, in months 12 months
Number of purchase periods allowed under the plan 2pre_SharePurchasePlansNumberOfPurchasePeriods
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= pre_EmployeeSharePurchasePlanMember
Length of purchase periods, in months 6 months
Contribution range low 1.00%pre_SharePurchasePlansContributionRateLow
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= pre_EmployeeSharePurchasePlanMember
Contribution range high 10.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= pre_EmployeeSharePurchasePlanMember
Discount on fair value 15.00%pre_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPriceOnOfferingOrPurchaseDate
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= pre_EmployeeSharePurchasePlanMember
Number of shares allocated to plan 0.6us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= pre_EmployeeSharePurchasePlanMember
Shares remaining for issuance 0.2us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= pre_EmployeeSharePurchasePlanMember
Swiss Share Purchase Plan [Member]  
Employee Share Purchase Plans [Line Items]  
Number of purchase periods allowed under the plan 2pre_SharePurchasePlansNumberOfPurchasePeriods
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= pre_SwissSharePurchasePlanMember
Length of purchase periods, in months 6 months
Contribution range low 1.00%pre_SharePurchasePlansContributionRateLow
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= pre_SwissSharePurchasePlanMember
Contribution range high 8.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= pre_SwissSharePurchasePlanMember
Contribution limit maximum, in CHF 5,000pre_SharePurchasePlansMaximumContributionPerEmployee
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= pre_SwissSharePurchasePlanMember
Discount on fair value 40.00%pre_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPriceOnOfferingOrPurchaseDate
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= pre_SwissSharePurchasePlanMember
Restriction on transfer, in years 2 years
Number of shares allocated to plan 0.4us-gaap_EmployeeStockOwnershipPlanESOPSharesInESOP
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= pre_SwissSharePurchasePlanMember
Shares remaining for issuance 0.2us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= pre_SwissSharePurchasePlanMember
Minimum Working Hours To Qualify 20pre_MinimumWorkingHoursToQualify
/ us-gaap_EmployeeStockOwnershipPlanESOPDisclosuresByPlanAxis
= pre_SwissSharePurchasePlanMember
Share Options and SSARs [Member]  
Employee Share Purchase Plans [Line Items]  
Shares remaining for issuance 2.7us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_AwardTypeAxis
= pre_ShareOptionsAndShareSettledShareAppreciationRightsMember
Non-Employee Directors [Member]  
Non-employee directors stock plan [Line Items]  
Number of share-based awards authorized 1.2us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSharesApprovedForIssuance
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= pre_NonEmployeeDirectorsMember
Shares remaining for issuance 0.3pre_ShareBasedGoodsAndNonemployeeServicesTransactionNumberOfSharesAvailableForGrant
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= pre_NonEmployeeDirectorsMember
Non-Employee Directors [Member] | RSU [Member]  
Non-employee directors stock plan [Line Items]  
Number of share-based awards authorized 0.8us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSharesApprovedForIssuance
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= pre_NonEmployeeDirectorsMember
Non-Employee Directors [Member] | Share Options and SSARs [Member]  
Non-employee directors stock plan [Line Items]  
Number of share-based awards authorized 0.4us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSharesApprovedForIssuance
/ us-gaap_AwardTypeAxis
= pre_ShareOptionsAndShareSettledShareAppreciationRightsMember
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= pre_NonEmployeeDirectorsMember
Non-Employee Directors [Member] | Share Options and SSARs granted prior to May 2012 [Member] | Share Options and SSARs [Member]  
Non-employee directors stock plan [Line Items]  
Contractual term, in years 10 years
Non-Employee Directors [Member] | Share Options and SSARs granted after to May 2012 [Member] | Share Options and SSARs [Member]  
Non-employee directors stock plan [Line Items]  
Contractual term, in years 10 years
Vesting period, in years 3 years
Non-Employee Directors [Member] | RSU Granted Prior To May 2010 [Member] | RSU [Member]  
Non-employee directors stock plan [Line Items]  
Delivery date restrictions, in years 1 year
Non-Employee Directors [Member] | RSU Granted Post May 2010 [Member] | RSU [Member]  
Non-employee directors stock plan [Line Items]  
Vesting period, in years 5 years
Delivery date restrictions, in years 0 years
Directors Percentage of RSUs Elected 60.00%pre_NonEmployeeShareBasedTransactionAwardPercentageRestrictedStockUnitsElected
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ pre_NonEmployeeDirectorRsuAwardsByGrantDateAxis
= pre_RsuGrantedPostMay2010Member
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= pre_NonEmployeeDirectorsMember
Directors Percentage Cash Elected In Lieu Of RSUs 40.00%pre_NonEmployeeShareBasedTransactionAwardPercentageCashElectedInLieuOfRestrictedStockUnits
/ us-gaap_AwardTypeAxis
= us-gaap_RestrictedStockUnitsRSUMember
/ pre_NonEmployeeDirectorRsuAwardsByGrantDateAxis
= pre_RsuGrantedPostMay2010Member
/ us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis
= pre_NonEmployeeDirectorsMember