Name:	pfg_AdditionalLiabilityForCertainBenefitFeaturesActuarialAssumptions
Namespace Prefix:	pfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Amount recoverable under reinsurance for present value of total expected payment to policyholder in excess of present value of total expected assessment. Excludes benefit classified as market risk benefit or under provisions of Topic 815 on derivative and hedging.

No definition available.

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

Amount of increase/decrease to income before taxes, net of reinsurance, from actuarial assumptions.

No definition available.

Name:	pfg_IncomeBeforeTaxesNetOfReinsuranceActuarialAssumptions
Namespace Prefix:	pfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Amount, after effect of reinsurance, of present value of total expected payment to policyholder in excess of present value of total expected assessment. Excludes benefit classified as market risk benefit or under provisions of Topic 815 on derivative and hedging.

Name:	us-gaap_AdditionalLiabilityLongDurationInsuranceAfterReinsurance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of cash outflow to policyholder related to benefit payment for present value of total expected payment to policyholder in excess of present value of total expected assessment. Excludes benefit classified as market risk benefit or under provisions of Topic 815 on derivative and hedging.

Name:	us-gaap_AdditionalLiabilityLongDurationInsuranceBenefitPayment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Amount of cumulative increase (decrease) in present value of total expected payment to policyholder in excess of present value of total expected assessment from cash flow assumption change for future cash flows. Excludes benefit classified as market risk benefit or under provisions of Topic 815 on derivative and hedging.

Name:	us-gaap_AdditionalLiabilityLongDurationInsuranceCumulativeIncreaseDecreaseFromCashFlowChange
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of cumulative increase (decrease) in present value of total expected payment to policyholder in excess of present value of total expected assessment from effect of variance from cash flow assumption change for actual experience. Excludes benefit classified as market risk benefit or under provisions of Topic 815 on derivative and hedging.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (b)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-6

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 13I
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480046/944-40-55-13I

Name:	us-gaap_AdditionalLiabilityLongDurationInsuranceCumulativeIncreaseDecreaseOfActualVarianceFromExpectedExperience
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of interest income (expense) for present value of total expected payment to policyholder in excess of present value of total expected assessment. Excludes benefit classified as market risk benefit or under provisions of Topic 815 on derivative and hedging.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-6

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-6

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-6

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 13I
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480046/944-40-55-13I

Name:	us-gaap_AdditionalLiabilityLongDurationInsuranceInterestIncomeExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Amount of gross premium collected from policyholder to fund expected benefit payment for present value of total expected payment to policyholder in excess of present value of total expected assessment. Excludes benefit classified as market risk benefit or under provisions of Topic 815 on derivative and hedging.

Name:	us-gaap_AdditionalLiabilityLongDurationInsuranceNetPremiumCollected
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount, before effect of reinsurance and current period update of cash flow assumption, of present value of total expected payment to policyholder in excess of present value of total expected assessment, discounted at original rate. Excludes benefit classified as market risk benefit or under provisions of Topic 815 on derivative and hedging.

Name:	us-gaap_AdditionalLiabilityLongDurationInsuranceOriginalDiscountRateBeforeCashFlowAndReinsurance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount, before effect of reinsurance and after current period update of cash flow assumption, of present value of total expected payment to policyholder in excess of present value of total expected assessment, discounted at original rate. Excludes benefit classified as market risk benefit or under provisions of Topic 815 on derivative and hedging.

Name:	us-gaap_AdditionalLiabilityLongDurationInsuranceOriginalDiscountRateBeforeReinsuranceAfterCashFlowChange
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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

Weighted-average duration of present value of total expected payment to policyholder in excess of present value of total expected assessment, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Excludes benefit classified as market risk benefit or under provision of Topic 815 on derivative and hedging.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (b)(5)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-6