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Debt (Details 5) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Non-recourse mortgages and notes payable    
Long-Term Debt    
Outstanding principal balances per real estate development, low end of range $ 1.7pfg_DebtInstrumentCarryingAmountLowEndOfRange
/ us-gaap_DebtInstrumentAxis
= pfg_NonRecourseMortgagesAndNotesPayableMember		 $ 1.6pfg_DebtInstrumentCarryingAmountLowEndOfRange
/ us-gaap_DebtInstrumentAxis
= pfg_NonRecourseMortgagesAndNotesPayableMember
Outstanding principal balances per real estate development, high end of range 32.6pfg_DebtInstrumentCarryingAmountHighEndOfRange
/ us-gaap_DebtInstrumentAxis
= pfg_NonRecourseMortgagesAndNotesPayableMember		 20.1pfg_DebtInstrumentCarryingAmountHighEndOfRange
/ us-gaap_DebtInstrumentAxis
= pfg_NonRecourseMortgagesAndNotesPayableMember
Interest rates per real estate development (as a percent)   5.50%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= pfg_NonRecourseMortgagesAndNotesPayableMember
Real estate 155.6pfg_InvestmentsInRealEstate
/ us-gaap_DebtInstrumentAxis
= pfg_NonRecourseMortgagesAndNotesPayableMember		 101.4pfg_InvestmentsInRealEstate
/ us-gaap_DebtInstrumentAxis
= pfg_NonRecourseMortgagesAndNotesPayableMember
Other mortgages and notes payable    
Long-Term Debt    
Long-term debt obligation assumed with purchase of WM Advisors, Inc.   $ 0.2pfg_LongTermDebtAssumedWithPurchaseOfWMAdvisorsInc
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= pfg_OtherMortgagesAndNotesPayableMember