Name:	pl_DefinedBenefitPlanAdditionalContributionByEmployerForCurrentPlanYear
Namespace Prefix:	pl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Represents the maximum adjusted funding target attainment percentage below which a plan could be subject to certain benefit restrictions under the Pension Protection Act of 2006.

No definition available.

Name:	pl_DefinedBenefitPlanAdjustedFundingTargetAttainmentPercentageSubjectToBenefitRestrictionsMaximum
Namespace Prefix:	pl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Represents the adjusted funding target attainment percentage to be maintained by the entity.

No definition available.

Name:	pl_DefinedBenefitPlanAdjustedFundingTargetAttainmentPercentageToBeMaintained
Namespace Prefix:	pl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

The increase in the fair value of plan assets as a result of contributions made by the employer for the current fiscal year.

No definition available.

Name:	pl_DefinedBenefitPlanContributionsByEmployerForCurrentPlanYear
Namespace Prefix:	pl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The increase in the fair value of plan assets as a result of contributions made by the employer for the prior plan year.

No definition available.

Name:	pl_DefinedBenefitPlanContributionsByEmployerForPriorPlanYear
Namespace Prefix:	pl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The employer's best estimate, as soon as it can be reasonably determined, of contributions expected to be paid to the plan during the next fiscal year, in addition to contributions already made after the date of the latest statement of financial position. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions.

No definition available.

Name:	pl_DefinedBenefitPlanEstimatedFutureEmployerContributionsInRemainderOfNextFiscalYear
Namespace Prefix:	pl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Represents the threshold age plus vesting period of active employees, below which employees are eligible for a final pay-based pension benefit for service through December 31, 2007 and a cash balance benefit for service thereafter, and above which employees are eligible for a final pay-based pension benefit for service both before and after December 31, 2007, with a modest reduction in the formula for benefits earned thereafter.

No definition available.