EMPLOYEE BENEFIT PLANS - Defined Benefit Pension Plan and Unfunded Excess Benefit Plan (Details)	12 Months Ended
Dec. 31, 2018
Qualified Plan \| Qualified Pension Plan
EMPLOYEE BENEFIT PLANS
Threshold age plus vesting period of active employees to determine type of benefit eligibility	55 years

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.

Name:	pl_PayBasedPensionBenefitForServiceEligibleActiveEmployeesAgePlusVestingPeriodThreshold
Namespace Prefix:	pl_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration