Debt - Short-Term Debt (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Short-Term Debt
Credit facilities	$ 986.6	$ 994.5
Short-term debt	$ 84.7	$ 93.4
Weighted-average interest rates on short-term borrowings (as a percent)	3.40%	3.40%
Credit facility maturing November 2023 \| PFG, PFS, Principal Life as co-borrowers
Short-Term Debt
Credit facilities	$ 600.0	$ 600.0
Credit facility maturing November 2023 \| PFG, PFS, Principal Life and Principal Financial Services V (UK) LTD as co-borrowers
Short-Term Debt
Credit facilities	200.0	200.0
Unsecured lines of credit \| Principal International Chile
Short-Term Debt
Credit facilities	186.6	134.5
Short-term debt	$ 84.7	$ 93.4
Unsecured lines of credit \| Maximum \| Principal International Chile
Short-Term Debt
Term of credit facility	1 year	1 year
Unsecured line of credit maturing September 2020 \| Principal Life Insurance Company
Short-Term Debt
Credit facilities		$ 60.0
Commercial paper
Short-Term Debt
Support provided by back-stop facility for commercial paper program (as a percent)	100.00%	100.00%
Back-stop facility outstanding balances	$ 0.0	$ 0.0

Amount of outstanding back-stop facilities as of the balance sheet date.

No definition available.

Name:	pfg_BackStopFacilityOutstandingBalances
Namespace Prefix:	pfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Percentage of support provided by back-stop facility for commercial paper program.

+ References

No definition available.

+ Details

Name:	pfg_BackStopFacilitySupportForCommercialPaperProgramPercent
Namespace Prefix:	pfg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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

- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16)(a)(1))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

- Definition

Weighted average interest rate of short-term debt outstanding calculated at point in time.

+ References

+ Details

Name:	us-gaap_ShortTermDebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=pfg_CreditFacilityMaturingNovember2023Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	pfg_ParentAndSubsidiariesShelfRegistrationDebtGuarantorAxis=pfg_PrincipalFinancialServicesIncAndPrincipalLifeInsuranceCompanyAndPrincipalFinancialGroupCoborrowersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	pfg_ParentAndSubsidiariesShelfRegistrationDebtGuarantorAxis=pfg_PrincipalFinancialServicesIncAndPrincipalLifeInsuranceCompanyAndPrincipalFinancialServicesVLTDAndPrincipalFinancialGroupCoborrowersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=pfg_UnsecuredLineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	pfg_ParentAndSubsidiariesShelfRegistrationDebtGuarantorAxis=pfg_PrincipalInternationalChileMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=pfg_UnsecuredLineOfCreditMaturingSeptember2020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	pfg_ParentAndSubsidiariesShelfRegistrationDebtGuarantorAxis=pfg_PrincipalLifeInsuranceCompanyConsolidatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_CommercialPaperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: