Accumulated Other Comprehensive Income (Loss), Adjustment of Net Unrealized Losses Reported

No definition available.

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

- Definition

Tax Cuts and Jobs Act of 2017, Reclassification from AOCI to Retained Earnings, Tax Effect

+ References

No definition available.

+ Details

Name:	pl_TaxCutsandJobsActof2017ReclassificationfromAOCItoRetainedEarningsTaxEffect
Namespace Prefix:	pl_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_AOCIAttributableToParentNetOfTaxRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 55
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=84158357&loc=d3e1436-108581

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580

+ Details

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

- Definition

Amount after tax of other comprehensive income (loss) attributable to parent entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=84161450&loc=SL4573702-111684

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 19
-URI http://asc.fasb.org/extlink&oid=96868048&loc=SL4569616-111683

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=96868048&loc=SL4569643-111683

+ Details

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

- Definition

Amount after tax of other than temporary impairment loss (OTTI) on investments in available-for-sale and held-to-maturity debt securities, recognized in other comprehensive loss and attributable to parent entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
-Paragraph 34D
-URI http://asc.fasb.org/extlink&oid=82895926&loc=SL6284304-111560

+ Details

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

- Definition

Amount after tax of reclassification adjustments of other comprehensive income (loss) attributable to parent.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 55
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=84158357&loc=d3e1436-108581

+ Details

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

- Definition

Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

+ 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(30))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 4.E)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74512-122707

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(31))
-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 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedOtherComprehensiveIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Changes in Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			$ 7,127,199
Total other comprehensive income (loss)	$ (429,405)	$ 293,525	(1,005,849)	$ 455,258
Ending Balance	6,044,432		6,044,432		$ 7,127,199
Offset of net unrealized losses in AOCI due to impact those net unrealized losses would have on certain of the Company's insurance assets and liabilities had the net unrealized losses been recognized in net income			361,600		(6,300)	$ 424,100
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			12,696	(654,523)	(654,523)
Other comprehensive income (loss) before reclassifications			(1,002,431)		684,247
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings			(3,691)		7,153
Amounts reclassified from accumulated other comprehensive income (loss)			273		1,594
Total other comprehensive income (loss)			(1,005,849)		692,994
Cumulative effect adjustments			(10,552)		(25,775)
Ending Balance	(1,003,705)		(1,003,705)		12,696	(654,523)
Unrealized Gains and Losses on Investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			25,874	(656,322)	(656,322)
Other comprehensive income (loss) before reclassifications			(1,005,262)		700,536
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings			(3,691)		7,153
Amounts reclassified from accumulated other comprehensive income (loss)			22		642
Total other comprehensive income (loss)			(1,008,931)		708,331
Cumulative effect adjustments			(10,552)		(26,135)
Ending Balance	(993,609)		(993,609)		25,874	(656,322)
Accumulated Gain and Loss Derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			747	727	727
Other comprehensive income (loss) before reclassifications			2,831		(563)
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings			0		0
Amounts reclassified from accumulated other comprehensive income (loss)			251		451
Total other comprehensive income (loss)			3,082		(112)
Cumulative effect adjustments			0		132
Ending Balance	3,829		3,829		747	727
Minimum Pension Liability Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance			(13,925)	$ 1,072	1,072
Other comprehensive income (loss) before reclassifications			0		(15,726)
Other comprehensive income (loss) relating to other-than-temporary impaired investments for which a portion has been recognized in earnings			0		0
Amounts reclassified from accumulated other comprehensive income (loss)			0		501
Total other comprehensive income (loss)			0		(15,225)
Cumulative effect adjustments			0		228
Ending Balance	$ (13,925)		$ (13,925)		$ (13,925)	$ 1,072