Derivative Financial Instruments - Fair Value Hedges (Details) - Fixed maturities, available-for-sale - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Fair Value Hedges
Amortized cost of hedged item - Active hedging relationships	$ 143.3	$ 137.0
Amortized cost of hedged item - Discontinued hedging relationships	248.7	298.9
Amortized cost of hedged item - Active or discontinued hedging relationships	392.0	435.9
Cumulative amount of fair value hedging basis adjustment - Active hedging relationships	19.0	12.4
Cumulative amount of fair value hedging basis adjustment - Discontinued hedging relationships	9.6	13.3
Cumulative amount of fair value hedging basis adjustment - Active or discontinued hedging relationships	$ 28.6	$ 25.7

Amount of asset previously hedged in a fair value hedging relationship that has been discontinued.

No definition available.

Name:	pfg_HedgedAssetDiscontinuedFairValueHedge
Namespace Prefix:	pfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of asset hedged in fair value hedging relationship or previously hedged in a fair value hedging relationship that has been discontinued.

+ References

No definition available.

+ Details

Name:	pfg_HedgedAssetFairValueHedgeIncludingDiscontinued
Namespace Prefix:	pfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk, including amount remaining after discontinued hedge.

+ References

No definition available.

+ Details

Name:	pfg_HedgedAssetFairValueHedgeIncludingDiscontinuedCumulativeIncreaseDecrease
Namespace Prefix:	pfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk, remaining after discontinued hedge.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109999712-113959

+ Details

Name:	us-gaap_HedgedAssetDiscontinuedFairValueHedgeCumulativeIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of asset hedged in fair value hedging relationship.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109999712-113959

+ Details

Name:	us-gaap_HedgedAssetFairValueHedge
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4EE
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109999712-113959

+ Details

Name:	us-gaap_HedgedAssetFairValueHedgeCumulativeIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_SummaryOfFairValueHedgeActivityAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_BalanceSheetLocationAxis=pfg_FixedMaturitiesSecuritiesAvailableForSaleMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: