Investments - AFS Securities with Cost Basis in Excess of Fair Value - Narrative (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
AFS fixed-maturity and equity securities with cost basis in excess of fair value, cost basis	$ 529.2	$ 450.9

Available for sale fixed maturity and equity securities with cost basis in excess of fair value cost basis.

No definition available.

Name:	pri_AvailableForSaleFixedMaturityAndEquitySecuritiesWithCostBasisInExcessOfFairValueCostBasis
Namespace Prefix:	pri_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant