MORTGAGE LOANS - Delinquent Loans (Details) - Commercial mortgage loans $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2017 USD ($) loan	Dec. 31, 2016 USD ($) loan
Delinquent loans
Past due period at which to cease carrying accrued interest on delinquent loans	90 days
Number of days accrued interest on impaired loans (less than)	90 days
Past due period at which to initiate foreclosure proceedings (more than)	90 days
Mortgage loans delinquent \| $	$ 8,245	$ 3,669
Number of mortgage loans delinquent \| loan	5	4
30 to 59 Days Delinquent
Delinquent loans
Mortgage loans delinquent \| $	$ 5,440	$ 3,669
Number of mortgage loans delinquent \| loan	3	4
60 to 89 Days Delinquent
Delinquent loans
Mortgage loans delinquent \| $	$ 0	$ 0
Number of mortgage loans delinquent \| loan	0	0
Greater than 90 Days Delinquent
Delinquent loans
Mortgage loans delinquent \| $	$ 2,805	$ 0
Number of mortgage loans delinquent \| loan	2	0

Represents the minimum number of days over which the entity's policy is to cease carrying accrued interest on loans.

No definition available.

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Namespace Prefix:	pl_
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Period Type:	duration

- Definition

Represents the minimum number of days over which the entity's policy is to initiate foreclosure proceedings.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of financing receivables that are past due at the balance sheet date.

+ References

No definition available.

+ Details

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

- Definition

The maximum number of days of accrued interest that can be carried on an impaired loan.

+ References

No definition available.

+ Details

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

- Definition

Recorded Investment in financing receivables that are past due at the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953401-111524

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

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- Details

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- Details

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- Details

Name:	us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables60To89DaysPastDueMember
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- Details

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