SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Guaranteed Living Withdrawal Benefits and Income Taxes (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($) subsidiary
Income Taxes
Number of subsidiaries not included in federal tax return \| subsidiary	1
Guaranteed minimum withdrawal benefits (GMWB) \| Annuity account
Guaranteed Minimum Withdrawal Benefits
Net GMWB liability \| $	$ 111.8

Represents the amount of expected guaranteed benefits in short of the projected account balances for minimum guaranteed benefit contracts, net of reinsurance recoverables, as of the balance sheet date.

No definition available.

Name:	pl_LiabilitiesForGuaranteesOnLongDurationContractsGuaranteedBenefitAssetNet
Namespace Prefix:	pl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant