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Investments (Details 8) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Dec. 31, 2013
Mortgage loan disclosures        
Mortgage loans, Total amortized cost $ 11,956.4us-gaap_NotesReceivableGross $ 11,544.9us-gaap_NotesReceivableGross $ 11,868.1us-gaap_NotesReceivableGross  
Mortgage loan valuation allowance (56.9)us-gaap_FinancingReceivableAllowanceForCreditLosses (66.5)us-gaap_FinancingReceivableAllowanceForCreditLosses (56.5)us-gaap_FinancingReceivableAllowanceForCreditLosses (69.8)us-gaap_FinancingReceivableAllowanceForCreditLosses
Mortgage loans, Total carrying value 11,899.5us-gaap_NotesReceivableNet   11,811.6us-gaap_NotesReceivableNet  
Commercial mortgage loans        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 10,820.8us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
10,299.0us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
10,723.8us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
 
Mortgage loan valuation allowance (27.8)us-gaap_FinancingReceivableAllowanceForCreditLosses
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
(26.5)us-gaap_FinancingReceivableAllowanceForCreditLosses
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
(26.9)us-gaap_FinancingReceivableAllowanceForCreditLosses
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
(28.7)us-gaap_FinancingReceivableAllowanceForCreditLosses
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
Mortgage loans, purchased 107.0us-gaap_FinancingReceivableSignificantPurchases
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
21.3us-gaap_FinancingReceivableSignificantPurchases
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
   
Mortgage loans, sold 1.0us-gaap_FinancingReceivableSignificantSales
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
1.1us-gaap_FinancingReceivableSignificantSales
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
   
Percent of mortgage loans (as a percent) 100.00%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
  100.00%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
 
Commercial mortgage loans | Office        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 3,798.7us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_OfficeBuildingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
  3,646.1us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_OfficeBuildingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
 
Percent of mortgage loans (as a percent) 35.10%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_OfficeBuildingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
  34.00%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_OfficeBuildingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
 
Commercial mortgage loans | Retail        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 2,562.7us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_RetailSiteMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
  2,512.1us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_RetailSiteMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
 
Percent of mortgage loans (as a percent) 23.70%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_RetailSiteMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
  23.40%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_RetailSiteMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
 
Commercial mortgage loans | Industrial        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 1,852.8us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_IndustrialPropertyMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
  1,918.7us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_IndustrialPropertyMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
 
Percent of mortgage loans (as a percent) 17.10%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_IndustrialPropertyMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
  17.90%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_IndustrialPropertyMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
 
Commercial mortgage loans | Apartments        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 2,194.8us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_ApartmentBuildingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
  2,200.5us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_ApartmentBuildingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
 
Percent of mortgage loans (as a percent) 20.30%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_ApartmentBuildingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
  20.50%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_ApartmentBuildingMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
 
Commercial mortgage loans | Hotel        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 296.2us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_HotelMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
  331.5us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_HotelMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
 
Percent of mortgage loans (as a percent) 2.70%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_HotelMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
  3.10%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_HotelMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
 
Commercial mortgage loans | Mixed use/other        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 115.6us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_OtherPropertyMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
  114.9us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_OtherPropertyMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
 
Percent of mortgage loans (as a percent) 1.10%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_OtherPropertyMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
  1.10%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_OtherPropertyMember
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
 
Commercial mortgage loans | New England        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 531.4us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_NewEnglandMember
  528.0us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_NewEnglandMember
 
Percent of mortgage loans (as a percent) 4.90%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_NewEnglandMember
  4.90%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_NewEnglandMember
 
Commercial mortgage loans | Middle Atlantic        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 2,947.2us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_MiddleAtlanticMember
  2,951.0us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_MiddleAtlanticMember
 
Percent of mortgage loans (as a percent) 27.20%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_MiddleAtlanticMember
  27.50%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_MiddleAtlanticMember
 
Commercial mortgage loans | East North Central        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 471.8us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_EastNorthCentralMember
  442.1us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_EastNorthCentralMember
 
Percent of mortgage loans (as a percent) 4.40%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_EastNorthCentralMember
  4.10%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_EastNorthCentralMember
 
Commercial mortgage loans | West North Central        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 282.0us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_WestNorthCentralMember
  233.3us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_WestNorthCentralMember
 
Percent of mortgage loans (as a percent) 2.60%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_WestNorthCentralMember
  2.20%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_WestNorthCentralMember
 
Commercial mortgage loans | South Atlantic        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 1,977.2us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_SouthAtlanticMember
  1,970.9us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_SouthAtlanticMember
 
Percent of mortgage loans (as a percent) 18.30%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_SouthAtlanticMember
  18.40%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_SouthAtlanticMember
 
Commercial mortgage loans | East South Central        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 200.9us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_EastSouthCentralMember
  197.4us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_EastSouthCentralMember
 
Percent of mortgage loans (as a percent) 1.90%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_EastSouthCentralMember
  1.80%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_EastSouthCentralMember
 
Commercial mortgage loans | West South Central        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 1,062.8us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_WestSouthCentralMember
  1,023.9us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_WestSouthCentralMember
 
Percent of mortgage loans (as a percent) 9.80%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_WestSouthCentralMember
  9.50%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_WestSouthCentralMember
 
Commercial mortgage loans | Mountain        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 804.0us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_MountainMember
  772.0us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_MountainMember
 
Percent of mortgage loans (as a percent) 7.40%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_MountainMember
  7.20%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_MountainMember
 
Commercial mortgage loans | Pacific        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 2,505.2us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_PacificMember
  2,565.5us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_PacificMember
 
Percent of mortgage loans (as a percent) 23.10%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_PacificMember
  24.00%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_PacificMember
 
Commercial mortgage loans | International        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 38.3us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_InternationalMember
  39.7us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_InternationalMember
 
Percent of mortgage loans (as a percent) 0.40%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_InternationalMember
  0.40%pfg_PercentageOfMortgageLoans
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
/ us-gaap_StatementGeographicalAxis
= pfg_InternationalMember
 
Residential mortgage loans        
Mortgage loan disclosures        
Mortgage loans, Total amortized cost 1,135.6us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember
1,245.9us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember
1,144.3us-gaap_NotesReceivableGross
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember
 
Mortgage loan valuation allowance (29.1)us-gaap_FinancingReceivableAllowanceForCreditLosses
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember
(40.0)us-gaap_FinancingReceivableAllowanceForCreditLosses
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember
(29.6)us-gaap_FinancingReceivableAllowanceForCreditLosses
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember
(41.1)us-gaap_FinancingReceivableAllowanceForCreditLosses
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember
Mortgage loans, purchased 63.5us-gaap_FinancingReceivableSignificantPurchases
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember
39.8us-gaap_FinancingReceivableSignificantPurchases
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember
   
Mortgage loans, sold 21.4us-gaap_FinancingReceivableSignificantSales
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember
0us-gaap_FinancingReceivableSignificantSales
/ us-gaap_MortgageLoansOnRealEstateLoanTypeAxis
= us-gaap_ResidentialMortgageMember
   
Home equity        
Mortgage loan 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
 
First liens        
Mortgage loan 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