Debt Instrument, Outstanding Principal

No definition available.

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

- Definition

Extinguishment of Debt Par Value

+ References

No definition available.

+ Details

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

- Definition

Maximum borrowing capacity under the credit facility which can be granted upon request by the entity.

+ References

No definition available.

+ Details

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

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
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

Represents the aggregate of total long-term debt, including current maturities and short-term debt.

+ References

No definition available.

+ Details

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

- Definition

Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DeferredFinanceCostsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=49170532&loc=d3e12317-112629

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=49170532&loc=d3e12355-112629

+ Details

Name:	us-gaap_GainsLossesOnExtinguishmentOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The fee, expressed as a percentage of the line of credit facility, for the line of credit facility regardless of whether the facility has been used.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityCommitmentFeePercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

The cash outflow during the period from the repayment of aggregate short-term and long-term debt. Excludes payment of capital lease obligations.

+ References

No definition available.

+ Details

Name:	us-gaap_RepaymentsOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_CapitalLeaseObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SubordinatedDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=pl_SeniorNotes6.40PercentDue2018Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=pl_SeniorNotes7.375PercentDue2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=pl_SeniorNotes8.45PercentDue2039Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=pl_SubordinatedDebentures5.35PercentDue2052Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=pl_SubordinatedDebentures3.55PercentDue2038OneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=pl_SubordinatedDebentures3.55PercentDue2038TwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=pl_SubordinatedDebentures3.55PercentDue2038Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=pl_SubordinatedDebentures6.00PercentDue2042Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=pl_SubordinatedDebentures6.25PercentDue2042Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=pl_AmendedLineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_BaseRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=pl_DebtInstrumentVariableRateBaseLIBOROneMonthMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=pl_A2015CreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

DEBT AND OTHER OBLIGATIONS - Summary of Debt and Subordinated Debt Securities (Details) - USD ($)		6 Months Ended	12 Months Ended
	May 03, 2018	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Outstanding Principal		$ 980,745,000	$ 784,610,000
Carrying Amounts, Debt		1,109,049,000	945,052,000
Carrying Amounts, Subordinated debt securities		605,358,000	495,289,000
Credit Facility
Debt Instrument [Line Items]
Outstanding Principal		385,000,000	0
Carrying Amounts, Debt		385,000,000	0
Capital lease obligation
Debt Instrument [Line Items]
Outstanding Principal		1,501,000	1,682,000
Carrying Amounts, Debt		1,501,000	1,682,000
Subordinated debt securities
Debt Instrument [Line Items]
Outstanding Principal		610,000,000	500,000,000
Carrying Amounts, Subordinated debt securities		605,358,000	495,289,000
6.40% Senior Notes (2007), due 2018
Debt Instrument [Line Items]
Outstanding Principal		0	150,000,000
Carrying Amounts, Debt		0	$ 150,518,000
Stated interest rate (as a percent)			6.40%
7.375% Senior Notes (2009), due 2019
Debt Instrument [Line Items]
Outstanding Principal		400,000,000	$ 400,000,000
Carrying Amounts, Debt		$ 426,195,000	435,806,000
Stated interest rate (as a percent)		7.375%
8.45% Senior Notes (2009), due 2039
Debt Instrument [Line Items]
Outstanding Principal		$ 194,244,000	232,928,000
Carrying Amounts, Debt		296,353,000	357,046,000
Debt repurchased and subsequently extinguished		59,200,000
Par value of extinguished debt		$ 38,700,000
Stated interest rate (as a percent)		8.45%
Pre-tax gain (loss) on extinguishment of debt		$ 1,500,000
5.35% Subordinated Debentures (2017), due 2052
Debt Instrument [Line Items]
Stated interest rate (as a percent)		5.35%
5.35% Subordinated Debentures (2017), due 2052 \| Subordinated debt securities
Debt Instrument [Line Items]
Outstanding Principal		$ 500,000,000	500,000,000
Carrying Amounts, Subordinated debt securities		495,358,000	495,289,000
Deferred issuance expenses			4,800,000
3.55% Subordinated Funding Obligations (2018), due 2038 (One) \| Subordinated debt securities
Debt Instrument [Line Items]
Outstanding Principal		55,000,000	0
Carrying Amounts, Subordinated debt securities		$ 55,000,000	0
Stated interest rate (as a percent)		3.55%
3.55% Subordinated Funding Obligations (2018), due 2038 (Two) \| Subordinated debt securities
Debt Instrument [Line Items]
Outstanding Principal		$ 55,000,000	0
Carrying Amounts, Subordinated debt securities		$ 55,000,000	$ 0
Stated interest rate (as a percent)		3.55%
3.55% Subordinated Funding Obligations (2018), due 2038
Debt Instrument [Line Items]
Stated interest rate (as a percent)		3.55%
3.55% Subordinated Funding Obligations (2018), due 2038 \| Subordinated debt securities
Debt Instrument [Line Items]
Outstanding Principal		$ 110,000,000
6.00% suordinated debentures due 2042
Debt Instrument [Line Items]
Stated interest rate (as a percent)			6.00%
6.00% suordinated debentures due 2042 \| Subordinated debt securities
Debt Instrument [Line Items]
Debt repurchased and subsequently extinguished			$ 150,000,000
6.25% subordinated debentures due 2042
Debt Instrument [Line Items]
Stated interest rate (as a percent)			6.25%
6.25% subordinated debentures due 2042 \| Subordinated debt securities
Debt Instrument [Line Items]
Debt repurchased and subsequently extinguished			$ 287,500,000
Credit Facility
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	$ 1,000,000,000.0
Line of credit, maximum borrowing capacity to be granted upon entity's request	$ 1,500,000,000.0
Facility fee percentage	0.125%
Credit Facility \| Federal Funds Rate
Debt Instrument [Line Items]
Interest rate added to the base rate (as a percent)	0.50%
Credit Facility \| One-Month LIBOR
Debt Instrument [Line Items]
Interest rate added to the base rate (as a percent)	1.00%
Credit Facility \| LIBOR
Debt Instrument [Line Items]
Interest rate added to the base rate (as a percent)		1.00%
2015 Credit Facility
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	$ 1,000,000,000.0
Line of credit, maximum borrowing capacity to be granted upon entity's request	$ 1,250,000,000.00
Facility fee percentage	0.125%
2015 Credit Facility \| Federal Funds Rate
Debt Instrument [Line Items]
Interest rate added to the base rate (as a percent)	0.50%
2015 Credit Facility \| One-Month LIBOR
Debt Instrument [Line Items]
Interest rate added to the base rate (as a percent)	1.00%