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Investments (Details 9) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost $ 11,956.4us-gaap_NotesReceivableGross $ 11,868.1us-gaap_NotesReceivableGross $ 11,544.9us-gaap_NotesReceivableGross
Commercial mortgage loans      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 10,820.8us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 10,723.8us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 10,299.0us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
Commercial mortgage loans | A- and above      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 9,371.9us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsAMinusAndAboveRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 9,284.6us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsAMinusAndAboveRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		  
Commercial mortgage loans | BBB+ thru BBB-      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 1,224.2us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBBPlusThruBBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 1,219.5us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBBPlusThruBBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		  
Commercial mortgage loans | BB+ thru BB-      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 166.1us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBPlusThruBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 148.3us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBPlusThruBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		  
Commercial mortgage loans | B+ and below      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 58.6us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBPlusAndBelowRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 71.4us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBPlusAndBelowRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		  
Brick and mortar      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 10,486.3us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		 10,374.9us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		  
Brick and mortar | A- and above      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 9,185.9us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsAMinusAndAboveRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		 9,115.8us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsAMinusAndAboveRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		  
Brick and mortar | BBB+ thru BBB-      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 1,077.2us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBBPlusThruBBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		 1,041.0us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBBPlusThruBBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		  
Brick and mortar | BB+ thru BB-      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 166.1us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBPlusThruBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		 148.3us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBPlusThruBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		  
Brick and mortar | B+ and below      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 57.1us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBPlusAndBelowRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		 69.8us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBPlusAndBelowRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		  
Credit tenant loans      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 334.5us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialCreditTenantMortgageMember		 348.9us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialCreditTenantMortgageMember		  
Credit tenant loans | A- and above      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 186.0us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsAMinusAndAboveRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialCreditTenantMortgageMember		 168.8us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsAMinusAndAboveRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialCreditTenantMortgageMember		  
Credit tenant loans | BBB+ thru BBB-      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 147.0us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBBPlusThruBBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialCreditTenantMortgageMember		 178.5us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBBPlusThruBBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialCreditTenantMortgageMember		  
Credit tenant loans | B+ and below      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 1.5us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBPlusAndBelowRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialCreditTenantMortgageMember		 1.6us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBPlusAndBelowRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialCreditTenantMortgageMember		  
Residential mortgage loans      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 1,135.6us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember		 1,144.3us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember		 1,245.9us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember
Residential mortgage loans | Performing      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 1,108.9us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_PerformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember		 1,116.0us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_PerformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember		  
Residential mortgage loans | Nonperforming      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 26.7us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_NonperformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember		 28.3us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_NonperformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember		  
Mortgage loans, Days delinquent to be considered nonperforming 90 days 90 days  
Home equity      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 268.1us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_HomeEquityMember		 283.4us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_HomeEquityMember		  
Home equity | Performing      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 254.9us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_PerformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_HomeEquityMember		 268.4us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_PerformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_HomeEquityMember		  
Home equity | Nonperforming      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 13.2us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_NonperformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_HomeEquityMember		 15.0us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_NonperformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_HomeEquityMember		  
First liens      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 867.5us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_FirstMortgageMember		 860.9us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_FirstMortgageMember		  
First liens | Performing      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 854.0us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_PerformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_FirstMortgageMember		 847.6us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_PerformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_FirstMortgageMember		  
First liens | Nonperforming      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost $ 13.5us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_NonperformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_FirstMortgageMember		 $ 13.3us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_NonperformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_FirstMortgageMember