DEBT AND OTHER OBLIGATIONS - Repurchase Program Borrowings (Details) - Repurchase Program Borrowings - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Fair value of securities pledged under the repurchase program	$ 457.5	$ 1,006.6
Secured financing liabilities	$ 417.1	$ 885.0
Average borrowing rate (as a percent)	2.23%	1.42%
Maximum balance outstanding	$ 427.5	$ 988.5
Average daily balance	$ 225.9	$ 624.7
Average daily balance, average borrowing rate (as a percent)	2.08%	1.01%
Maximum
Debt Instrument [Line Items]
Term of financing agreement	90 days

Represents the average daily balance of reverse repurchase liability.

No definition available.

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

- Definition

Represents the average borrowing rate of average daily balance of the repurchase obligation.

+ References

No definition available.

+ Details

Name:	pl_ReverseRepurchaseAverageDailyBalanceAtAverageInterestRate
Namespace Prefix:	pl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents proceeds from reverse repurchase liability for the balance sheet date.

+ References

No definition available.

+ Details

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

- Definition

Represents the average borrowing rate of repurchase obligation.

+ References

No definition available.

+ Details

Name:	pl_ReverseRepurchaseLiabilityAverageInterestRate
Namespace Prefix:	pl_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the maximum outstanding balance of reverse repurchase liability.

+ References

No definition available.

+ Details

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

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The fair value, as of the date of each statement of financial position presented, of securities which are owned but transferred to serve as collateral for the payment of repurchase agreements, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

+ Details

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

- Details

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

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: