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Goodwill and Other Intangible Assets (Details 3) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Mar. 31, 2013
Indefinite Lived Intangible Assets      
Net carrying amount of unamortized indefinite lived intangible assets $ 833.0us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill $ 856.3us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill  
WM Advisors, Inc.      
Indefinite Lived Intangible Assets      
Net carrying amount of unamortized indefinite lived intangible assets 608.0us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_BusinessAcquisitionAxis
= pfg_WMAdvisorsMember		 608.0us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_BusinessAcquisitionAxis
= pfg_WMAdvisorsMember		  
Cuprum | Trade names      
Indefinite Lived Intangible Assets      
Net carrying amount of unamortized indefinite lived intangible assets     $ 185.2us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill
/ us-gaap_BusinessAcquisitionAxis
= pfg_CuprumAcquisitionMember
/ us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis
= us-gaap_TradeNamesMember