EMPLOYEE BENEFIT PLANS (Details 9) (USD $)	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010
Other Postretirement Benefits \| Maximum
EMPLOYEE BENEFIT PLANS
Age of eligibility for limited healthcare benefits (in years)	65
Retiree medical plan
EMPLOYEE BENEFIT PLANS
Liability related to prescription drug benefit provided to employees	$ 100,000	$ 100,000
Retiree medical plan \| Minimum
EMPLOYEE BENEFIT PLANS
Age of eligibility for prescription drug benefit (in years)	65
Life insurance benefits	10,000
Retiree medical plan \| Maximum
EMPLOYEE BENEFIT PLANS
Life insurance benefits	75,000
Group life insurance plan
EMPLOYEE BENEFIT PLANS
Face amount of life insurance benefits funded	$ 50,000

Carrying amount as of the balance sheet date of the obligations recognized for the prescription drug benefit provided to retirees.

No definition available.

Name:	pl_DefinedBenefitPlanLiabilityForPostretirementMedicalPlansWithPrescriptionDrugBenefits
Namespace Prefix:	pl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Represents the minimum amount of life insurance benefit offered by the company.

No definition available.

Name:	pl_DefinedBenefitPlanLifeInsuranceBenefits
Namespace Prefix:	pl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Represents the face amount of life insurance benefits that is funded under the plan.

No definition available.

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

Represents the age of retired employees at which the entity provides limited healthcare benefits.

No definition available.

Name:	pl_DefinedBenefitPlanLimitedHealthcareBenefitsEligibilityAge
Namespace Prefix:	pl_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Represents the age of a closed group of retired employees at which the entity provides a prescription drug benefit.

No definition available.

Name:	pl_DefinedBenefitPlanPrescriptionDrugBenefitEligibilityAge
Namespace Prefix:	pl_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

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