Name:	pre_NonEmployeeShareBasedTransactionAwardElectionDetails
Namespace Prefix:	pre_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The discount rate at which employees may purchase the Company's shares. This is based on the fair value on the enrolment date or the purchase date, whichever is lower.

No definition available.

Name:	pre_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPriceOnOfferingOrPurchaseDate
Namespace Prefix:	pre_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Number of years of director services required for the award to vest.

No definition available.

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

The number of years for which there are restrictions on transfer of vested awards, from date of vesting.

No definition available.

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

The number of approved shares (or other type of equity) that are still available for issuance in the equity-based payment transaction as of the balance sheet date..

No definition available.

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

The low range percentage of employee's base salary that can be contributed to the share purchase plan.

No definition available.

Name:	pre_SharePurchasePlansContributionRateLow
Namespace Prefix:	pre_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

The length of the each offering period for share purchase plans, in months.

No definition available.

Name:	pre_SharePurchasePlansLengthOfOfferingPeriod
Namespace Prefix:	pre_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

The length of each purchase period for share purchase plans, in months.

No definition available.

Name:	pre_SharePurchasePlansLengthOfPurchasePeriod
Namespace Prefix:	pre_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

The maximum amount that may be contributed to the share purchase plan per annum, in CHF.

No definition available.

Name:	pre_SharePurchasePlansMaximumContributionPerEmployee
Namespace Prefix:	pre_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

The number of purchase periods for share purchase plans, per year.

No definition available.

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

The number of years that there is a restriction on sale or transfer following purchase, for shares purchased through share purchase plans.

No definition available.

Name:	pre_SharePurchasePlansRestrictionOnTransfer
Namespace Prefix:	pre_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Details

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

Sum of the allocated, committed-to-be-released and suspense shares of the entity held by the plan.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Statement of Position (SOP)

-Number 93-6

-Paragraph 53

-Subparagraph d

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 40

-Section 50

-Paragraph 1

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=6418621&loc=d3e17540-113929

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

The highest percentage of annual salary that an employee is permitted to utilize with respect to the plan.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph 65

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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Data Type:	num:percentItemType
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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Details

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

The maximum number of shares (or other type of equity) that have been approved for issuance in the equity-based payment transaction.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 50

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6406099&loc=d3e25284-112666

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)(3)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Name:	us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSharesApprovedForIssuance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

The period of time an equity-based award is expected to be outstanding. An equity-based award's expected term is generally determined based on, among other factors, the instrument's contractual term and the holders' expected behavior.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph 65

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 123R

-Paragraph A240

-Subparagraph e(2)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 505

-SubTopic 50

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6406099&loc=d3e25284-112666