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Debt (Details 4) (USD $)
In Millions, unless otherwise specified
0 Months Ended
Nov. 03, 2010
Dec. 31, 2014
Dec. 31, 2013
Mar. 01, 2014
Mar. 10, 1994
8% surplus notes payable, due 2044          
Long-Term Debt          
Long-term debt interest rate (as a percent)   8.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= pfg_SurplusNotesPayable8PercentDue2044Member
8.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= pfg_SurplusNotesPayable8PercentDue2044Member
   
3.76% notes payable, due 2015          
Long-Term Debt          
Long-term debt interest rate (as a percent) 3.76%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= pfg_NotesPayable3.76PercentDue2015Member
       
Principal International de Chile S.A. | 3.76% notes payable, due 2015          
Long-Term Debt          
Long-term debt issued $ 98.9us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= pfg_NotesPayable3.76PercentDue2015Member
/ dei_LegalEntityAxis
= pfg_PrincipalInternationalDeChileSaMember
       
Monetary unit used for denominating Chilean debt Unidades de Formento ("UF")        
Spread on UF interest rate basis for Chilean debt (as a percent) 3.76%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= pfg_NotesPayable3.76PercentDue2015Member
/ dei_LegalEntityAxis
= pfg_PrincipalInternationalDeChileSaMember
       
Principal Life Insurance Company | 8% surplus notes payable, due 2044          
Long-Term Debt          
Long-term debt issued         $ 100.0us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= pfg_SurplusNotesPayable8PercentDue2044Member
/ dei_LegalEntityAxis
= pfg_PrincipalLifeInsuranceCompanyConsolidatedMember
Long-term debt interest rate (as a percent)         8.00%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= pfg_SurplusNotesPayable8PercentDue2044Member
/ dei_LegalEntityAxis
= pfg_PrincipalLifeInsuranceCompanyConsolidatedMember
Approved redemption price of par (as a percent)       102.30%pfg_DebtInstrumentRedemptionApprovedRedemptionPrice
/ us-gaap_DebtInstrumentAxis
= pfg_SurplusNotesPayable8PercentDue2044Member
/ dei_LegalEntityAxis
= pfg_PrincipalLifeInsuranceCompanyConsolidatedMember