STOCK-BASED COMPENSATION (Details) (USD $) In Millions, except Share data, unless otherwise specified	6 Months Ended
Jun. 30, 2011
Performance awards
Stock-based compensation
Period over which average return on average equity is calculated (in years)	3
Minimum percentage of Company's ROE to earn awards under 2011 performance awards (as a percent)	9.00%
Minimum percentage of Company's ROE to earn maximum awards under 2011 performance awards (as a percent)	10.00%
Grant date fair values
Estimated fair values of awards at grant date	$ 5.4
Award activity
Awards granted (in shares)	191,000
SARs
Grant date fair values
Exercisable period of grants from grant date (in years)	five years
Minimum number of annual installments to exercise stock appreciation rights	3
Maximum number of annual installments to exercise stock appreciation rights	4
Minimum vesting period from the date of grants (in years)	one year
Term of award (in years)	ten years
Weighted-Average Base Price per share
Balance at the beginning of the period (in dollars per share)	$ 21.97
SARs exercised/forfeited/expired (in dollars per share)	$ 6.65
Balance at the end of the period (in dollars per share)	$ 22.25
Award activity
Balance at the beginning of the period (in shares)	2,324,837
Awards exercised / forfeited / expired (in shares)	41,319
Balance at the end of the period (in shares)	2,283,518
Restricted stock units
Grant date fair values
Estimated fair values of awards at grant date	$ 4.9
Award activity
Awards granted (in shares)	172,000

Represents the fair value at grant date for nonvested equity-based awards other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan), issued during the period.

Name:	pl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodGrantDateFairValue
Namespace Prefix:	pl
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of the period.

+ Details

Name:	pl_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsRollForward
Namespace Prefix:	pl
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the minimum percentage of average operating return on average equity for the entity, for the award maximum to be earned.

+ Details

Name:	pl_ShareBasedCompensationArrangementByShareBasedPaymentAwardsMinimumPercentageOfEntityROEToEarnMaximumAwards
Namespace Prefix:	pl
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the minimum percentage of average operating return on average equity for the entity, below which no award is earned.

+ Details

Name:	pl_ShareBasedCompensationArrangementByShareBasedPaymentAwardsMinimumPercentageOfROEToEarnAwards
Namespace Prefix:	pl
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	pl_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockAppreciationRightsAdditionalDisclosuresAbstract
Namespace Prefix:	pl
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of stock appreciation rights that were exercised, forfeited or expired during the period.

+ Details

Name:	pl_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockAppreciationRightsExercisesForfeituresAndExpirationsInPeriod
Namespace Prefix:	pl
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

The weighted-average base price of stock appreciation rights that were exercised, forfeited and expired during the reporting period.

+ Details

Name:	pl_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockAppreciationRightsForfeituresAndExpirationsInPeriodWeightedAverageBasePrice
Namespace Prefix:	pl
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of stock appreciation rights that validly exist and are outstanding as of the balance sheet date.

+ Details

Name:	pl_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockAppreciationRightsOutstandingNumber
Namespace Prefix:	pl
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted-average base price of outstanding stock appreciation rights as of the balance sheet date.

+ Details

Name:	pl_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockAppreciationRightsOutstandingWeightedAverageBasePrice
Namespace Prefix:	pl
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

The period after grant date upon which an employee's right to exercise an award begins to vest in annual installments.

+ Details

Name:	pl_ShareBasedCompensationArrangementByShareBasedPaymentAwardVestingInstallmentPeriodStart
Namespace Prefix:	pl
Data Type:	us-types:dateStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the period over which the entity's average operating return on average equity ("ROE") is calculated to determine the payment of awards.

+ Details

Name:	pl_ShareBasedCompensationArrangementByShareBasedPaymentPeriodForCalculationOfAverageReturnOnAverageEquity
Namespace Prefix:	pl
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the maximum number of annual installments in which an employee's right to exercise an award becomes no longer contingent on satisfaction of either a service condition, market condition or a performance condition.

+ Details

Name:	pl_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAnnualVestingInstallmentsMaximum
Namespace Prefix:	pl
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the minimum number of annual installments in which an employee's right to exercise an award becomes no longer contingent on satisfaction of either a service condition, market condition or a performance condition.

+ Details

Name:	pl_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAnnualVestingInstallmentsMinimum
Namespace Prefix:	pl
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The date when the equity-based award expires as specified in the award agreement, which may be presented in a variety of ways (for example, year, month and year, day, month and year, quarter of a year).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph a
-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 10
-Section 50
-Paragraph 2
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardExpirationDating
Namespace Prefix:	us-gaap
Data Type:	us-types:dateStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Description of the period of time over which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, which may be expressed in a variety of ways (for example, in years, month and year).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph a
-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 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod
Namespace Prefix:	us-gaap
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAdditionalDisclosuresAbstract
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph b(2)(c)
-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 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix:	us-gaap
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration