Adjustment relating to recovery of financing receivables.

No definition available.

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

- Definition

Represents the amount of nonperforming loans, foreclosed properties, and restructured loans pursuant to a pooling and servicing agreement.

+ References

No definition available.

+ Details

Name:	pl_FinancingReceivableAmountOfNonperformingForeclosedAndRestructuredLoans
Namespace Prefix:	pl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the number of categories of loans.

+ References

No definition available.

+ Details

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

- Definition

Represents the amount of financing receivables not subject to a pooling and servicing agreement that are nonperforming or have been restructured under the terms and conditions of the pooling and service agreement.

+ References

No definition available.

+ Details

Name:	pl_FinancingReceivableNotSubjectToPoolingAndServicingAgreementNonperformingOrRestructuredAmount
Namespace Prefix:	pl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the amount of financing receivables subject to a pooling and servicing agreement that are nonperforming or have been restructured under the terms and conditions of the pooling and service agreement.

+ References

No definition available.

+ Details

Name:	pl_FinancingReceivableSubjecttoPoolingandServicingAgreementNonperformingorRestructuredAmount
Namespace Prefix:	pl_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the loan-to-value ratio permitted on commercial mortgage loans in exchange for a participating interest in the cash flows from the underlying property.

+ References

No definition available.

+ Details

Name:	pl_MortgageLoansOnRealEstateLoanToValueRatioParticipatingInterest
Namespace Prefix:	pl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the amount of mortgage loans as of the balance sheet date which has a feature providing the entity a participating interest in the cash flows from the underlying real estate.

+ References

No definition available.

+ Details

Name:	pl_MortgageLoansOnRealEstateParticipatingLoans
Namespace Prefix:	pl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents the amount of income earned during the period on mortgage loans, which have a feature providing the entity a participating interest in the cash flows from the underlying real estate.

+ References

No definition available.

+ Details

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

- Definition

Number Of Nonperforming Financing Receivable Subject To Pooling And Servicing Agreement Which Have Been Restructured

+ References

No definition available.

+ Details

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

- Definition

Number Of Troubled Debt Restructurings

+ References

No definition available.

+ Details

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

- Definition

A valuation allowance for financing receivables that are expected to be uncollectible.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Amount of direct write-downs of financing receivables charged against the allowance.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

+ Details

Name:	us-gaap_ProvisionForLoanLeaseAndOtherLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=pl_CommercialMortgageLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

MORTGAGE LOANS - Commercial Mortgage Loans (Details)	3 Months Ended		9 Months Ended
MORTGAGE LOANS - Commercial Mortgage Loans (Details)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) restructuring category loan	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) loan	Dec. 31, 2016 USD ($)
Mortgage loans
Loans not subject to a pooling and servicing agreement which are either nonperforming or restructured					$ 0
Number of troubled debt restructurings \| restructuring			2
Number of nonperforming loans which have been restructured \| loan			0
Loans subject to a pooling and servicing agreement which are either nonperforming or restructured \| loan					0
Commercial mortgage loans
Mortgage loans
Mortgage loans having participation feature					$ 715,200,000	$ 595,200,000
Income recognized on participating mortgage loans	$ 14,200,000	$ 3,300,000	$ 25,700,000	$ 15,800,000
Loans not subject to a pooling and servicing agreement which are either nonperforming or restructured					3,100,000
Number of loan categories \| category			2
Nonperforming mortgage loans, foreclosed properties and restructured loans pursuant to pooling and servicing agreements					3,100,000
Allowance for mortgage loan credit losses	1,055,000		$ 724,000		$ 1,055,000	$ 724,000
Change in the allowance for credit losses
Beginning balance, December 31, 2016			724,000
Charge offs			(5,653,000)
Recoveries			(731,000)
Provision			6,715,000
Ending balance, September 30, 2017	$ 1,055,000		$ 1,055,000
Commercial mortgage loans \| Maximum
Mortgage loans
Loan-to-value ratio with participating interest (as a percent)					85.00%