DEBT AND OTHER OBLIGATIONS - Amount of Collateral Pledged for Repurchase Agreements and Securities Lending Transactions, Grouped by Asset Class (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Initial collateral (as a percent)	102.00%
Securities lending transactions	$ 88,989	$ 72,176
Fair value of collateral	94,700
Obligation to return securities	94,700
Repurchase agreements and repurchase-to-maturity transactions	92,872	451,895
Total securities	181,861	524,071
Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions	88,989	72,176
Repurchase agreements and repurchase-to-maturity transactions	83,512	433,182
Total securities	172,501	505,358
Up to 30 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Repurchase agreements and repurchase-to-maturity transactions	9,360	18,713
Total securities	9,360	18,713
30-90 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Repurchase agreements and repurchase-to-maturity transactions	0	0
Total securities	0	0
Greater Than 90 days
Debt Instrument [Line Items]
Securities lending transactions	0	0
Repurchase agreements and repurchase-to-maturity transactions	0	0
Total securities	0	0
U.S. Treasury and agency securities
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	92,872	451,895
U.S. Treasury and agency securities \| Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	83,512	433,182
U.S. Treasury and agency securities \| Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	9,360	18,713
U.S. Treasury and agency securities \| 30-90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
U.S. Treasury and agency securities \| Greater Than 90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions	0	0
Mortgage loans
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Mortgage loans \| Overnight and Continuous
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Mortgage loans \| Up to 30 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Mortgage loans \| 30-90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Mortgage loans \| Greater Than 90 days
Debt Instrument [Line Items]
Repurchase agreements and repurchase-to-maturity transactions		0
Corporate securities
Debt Instrument [Line Items]
Securities lending transactions	87,234
Corporate securities \| Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions	87,234
Corporate securities \| Up to 30 days
Debt Instrument [Line Items]
Securities lending transactions	0
Corporate securities \| 30-90 days
Debt Instrument [Line Items]
Securities lending transactions	0
Corporate securities \| Greater Than 90 days
Debt Instrument [Line Items]
Securities lending transactions	0
Fixed maturities
Debt Instrument [Line Items]
Securities lending transactions		71,285
Fixed maturities \| Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions		71,285
Fixed maturities \| Up to 30 days
Debt Instrument [Line Items]
Securities lending transactions		0
Fixed maturities \| 30-90 days
Debt Instrument [Line Items]
Securities lending transactions		0
Fixed maturities \| Greater Than 90 days
Debt Instrument [Line Items]
Securities lending transactions		0
Equity securities
Debt Instrument [Line Items]
Securities lending transactions		891
Equity securities \| Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions		891
Equity securities \| Up to 30 days
Debt Instrument [Line Items]
Securities lending transactions		0
Equity securities \| 30-90 days
Debt Instrument [Line Items]
Securities lending transactions		0
Equity securities \| Greater Than 90 days
Debt Instrument [Line Items]
Securities lending transactions		$ 0
Other government related securities
Debt Instrument [Line Items]
Securities lending transactions	1,755
Other government related securities \| Overnight and Continuous
Debt Instrument [Line Items]
Securities lending transactions	1,755
Other government related securities \| Up to 30 days
Debt Instrument [Line Items]
Securities lending transactions	0
Other government related securities \| 30-90 days
Debt Instrument [Line Items]
Securities lending transactions	0
Other government related securities \| Greater Than 90 days
Debt Instrument [Line Items]
Securities lending transactions	$ 0

Secured Borrowings, Percent Of Initial Collateral Required

No definition available.

Name:	pl_SecuredBorrowingsPercentOfInitialCollateralRequired
Namespace Prefix:	pl_
Data Type:	num:percentItemType
Balance Type:	na
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_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before effects of master netting arrangements, of funds outstanding borrowed in the form of a financial asset repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same financial asset at a date certain for a specified price. Includes repurchase-to-maturity transactions and liabilities not subject to a master netting arrangement.

+ References

+ Details

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

- Definition

Amount, before effects of master netting arrangements, of financial assets loaned to entities in exchange for collateral and funds outstanding borrowed in the form of a financial asset repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same financial assets at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 7
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL34722452-111729

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 7
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL34722452-111729

+ Details

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

- Definition

Amount of obligation to return securities collateral under master netting arrangements that have not been offset against securities borrowed.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 13
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226016-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (d)(2)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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

- Definition

Fair value of the securities received as collateral against securities borrowed.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 12
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226013-175313

+ Details

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

- Definition

Amount, before effects of master netting arrangements, of financial assets loaned to entities in exchange for collateral. Includes liabilities not subject to a master netting arrangement and financial assets transferred other than securities that qualify as secured borrowings.

+ References

+ Details

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

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=us-gaap_MaturityOvernightAndOnDemandMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=us-gaap_MaturityUpTo30DaysMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=us-gaap_Maturity30To90DaysMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetsSoldUnderAgreementsToRepurchaseMaturityPeriodsAxis=us-gaap_MaturityOver90DaysMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_LoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_FixedMaturitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=pl_OtherGovernmentRelatedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: