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Investments (Details 10) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost $ 11,868.1us-gaap_NotesReceivableGross $ 11,603.4us-gaap_NotesReceivableGross $ 11,617.1us-gaap_NotesReceivableGross
Commercial mortgage loans      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 10,723.8us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 10,327.7us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 10,235.1us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
Commercial mortgage loans | A- and above      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 9,284.6us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsAMinusAndAboveRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 8,286.4us-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 1,219.5us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBBPlusThruBBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 1,713.7us-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 148.3us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBPlusThruBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 155.5us-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 71.4us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBPlusAndBelowRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 172.1us-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 10,374.9us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		 9,881.1us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		  
Brick and mortar | A- and above      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 9,115.8us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsAMinusAndAboveRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		 8,091.9us-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 1,041.0us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBBPlusThruBBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		 1,463.7us-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 148.3us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBPlusThruBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		 155.4us-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 69.8us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBPlusAndBelowRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialBrickAndMortarMortgageMember		 170.1us-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 348.9us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialCreditTenantMortgageMember		 446.6us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialCreditTenantMortgageMember		  
Credit tenant loans | A- and above      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 168.8us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsAMinusAndAboveRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialCreditTenantMortgageMember		 194.5us-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 178.5us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBBPlusThruBBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialCreditTenantMortgageMember		 250.0us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBBPlusThruBBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialCreditTenantMortgageMember		  
Credit tenant loans | BB+ thru BB-      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost   0.1us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBBPlusThruBBMinusRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialCreditTenantMortgageMember		  
Credit tenant loans | B+ and below      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 1.6us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_StandardPoorsBPlusAndBelowRatingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= pfg_CommercialCreditTenantMortgageMember		 2.0us-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 1,144.3us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember		 1,275.7us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember		 1,382.0us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember
Residential mortgage loans | Performing      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 1,116.0us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_PerformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember		 1,240.4us-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 28.3us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_NonperformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember		 35.3us-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 283.4us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_HomeEquityMember		 394.9us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_HomeEquityMember		  
Home equity | Performing      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 268.4us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_PerformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_HomeEquityMember		 378.3us-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 15.0us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_NonperformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_HomeEquityMember		 16.6us-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 860.9us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_FirstMortgageMember		 880.8us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_FirstMortgageMember		  
First liens | Performing      
Mortgage loan credit quality disclosures      
Mortgage loans, Total amortized cost 847.6us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_PerformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_FirstMortgageMember		 862.1us-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 $ 13.3us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_NonperformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_FirstMortgageMember		 $ 18.7us-gaap_NotesReceivableGross
/ us-gaap_InternalCreditAssessmentAxis
= pfg_NonperformingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_FirstMortgageMember