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Segment Information (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Segment Information: Assets    
Total assets $ 219,087.0us-gaap_Assets $ 208,191.4us-gaap_Assets
Retirement and Investor Services    
Segment Information: Assets    
Total assets 138,549.4us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= pfg_SegmentRetirementAndInvestorServicesMember		 128,736.7us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= pfg_SegmentRetirementAndInvestorServicesMember
Principal Global Investors    
Segment Information: Assets    
Total assets 1,175.1us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= pfg_SegmentPrincipalGlobalInvestorsMember		 1,312.1us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= pfg_SegmentPrincipalGlobalInvestorsMember
Principal International    
Segment Information: Assets    
Total assets 53,531.8us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= pfg_SegmentPrincipalInternationalMember		 54,243.6us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= pfg_SegmentPrincipalInternationalMember
U.S. Insurance Solutions    
Segment Information: Assets    
Total assets 21,554.5us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= pfg_SegmentUSInsuranceSolutionsMember		 20,033.6us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= pfg_SegmentUSInsuranceSolutionsMember
Corporate    
Segment Information: Assets    
Total assets $ 4,276.2us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= pfg_SegmentCorporateMember		 $ 3,865.4us-gaap_Assets
/ us-gaap_StatementBusinessSegmentsAxis
= pfg_SegmentCorporateMember