Off-Balance Sheet Arrangements (Details) ([ForwardContractsMember], USD $) In Millions, except Per Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2010
[ForwardContractsMember]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Total number of shares of forward sale agreement	3.4
Extended forward agreement settlement	0
Valuation period	40
Valuation period - value low	$ 59.05
Valuation period - value high	$ 84.15

The maximum price per common share during the valuation period, in accordance with the extended forward sale agreement.

No definition available.

Name:	pre_ExtendedForwardAgreementSettlementPricePerShareHigh
Namespace Prefix:	pre_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

The minimum price per common share during the valuation period, in accordance with the extended forward sale agreement.

No definition available.

Name:	pre_ExtendedForwardAgreementSettlementPricePerShareLow
Namespace Prefix:	pre_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

The number of the Company's common shares under the extended forward sale agreement that matured on April 28, 2010.

No definition available.

Name:	pre_ExtendedForwardAgreementSettlementShares
Namespace Prefix:	pre_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

The number of days of the valuation period under which the Company could deliver the remaining number of Tranche 2 shares.

No definition available.

Name:	pre_ExtendedForwardAgreementSettlementValuationPeriod
Namespace Prefix:	pre_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

The total number of the Company's common shares under the forward sale agreement entered into with affiliate of Citigroup Capital Markets Inc.

No definition available.

Name:	pre_NumberOfSharesForwardAgreement
Namespace Prefix:	pre_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

Name:	us-gaap_FairValueOffBalanceSheetRisksDisclosureInformationLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration