Variable Interest Entities - Unconsolidated VIEs (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Money Market Funds
Total assets of unconsolidated money market mutual funds	$ 3,000.0	$ 4,400.0
Corporate debt securities \| Available-for-sale
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	235.3	244.2
Maximum exposure to loss	222.6	224.5
Residential mortgage-backed pass-through securities \| Available-for-sale
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	2,460.6	2,523.3
Maximum exposure to loss	2,488.5	2,493.8
Residential mortgage-backed pass-through securities \| Trading
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	322.6	366.5
Maximum exposure to loss	322.6	366.5
Commercial mortgage-backed securities \| Available-for-sale
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	3,945.6	3,708.3
Maximum exposure to loss	4,023.1	3,734.0
Commercial mortgage-backed securities \| Trading
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	13.8	0.7
Maximum exposure to loss	13.8	0.7
Collateralized debt obligations \| Available-for-sale
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	2,420.8	1,359.3
Maximum exposure to loss	2,451.3	1,372.1
Collateralized debt obligations \| Trading
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	11.8
Maximum exposure to loss	11.8
Other debt obligations \| Available-for-sale
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	7,153.2	5,646.2
Maximum exposure to loss	7,196.6	5,645.1
Other debt obligations \| Trading
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	9.7
Maximum exposure to loss	9.7
Equity securities
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	103.9	77.1
Maximum exposure to loss	103.9	77.1
Other investments: Other limited partnership and fund interests
Unconsolidated Variable Interest Entity disclosures
Asset carrying value	737.5	797.4
Maximum exposure to loss	1,432.2	$ 1,438.0
Fully secured debt of international real estate funds with recourse to the investment manager, included in maximum loss exposure	$ 132.2

No definition available.

Name:	pfg_MoneyMarketFundsAbstract
Namespace Prefix:	pfg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Total assets held by money market mutual funds that are not consolidated by the entity as the money market funds are registered under Rule 2a-7 of the Investment Company Act of 1940 or are similar unregistered funds.

+ References

No definition available.

+ Details

Name:	pfg_TotalAssetsUnconsolidatedMoneyMarketMutualFund
Namespace Prefix:	pfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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:	pfg_UnconsolidatedVariableInterestEntityLineItems
Namespace Prefix:	pfg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of debt of managed international rest estate funds that is fully secured by assets whose value exceeds the amount of the debt, but also includes recourse to the investment manager, included in the reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

+ References

No definition available.

+ Details

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

- Definition

The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5747-111685

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph b
-URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5728-111685

+ Details

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

- Definition

Carrying amount of the assets in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph a
-URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5728-111685

+ Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentSecondaryCategorizationAxis=us-gaap_AvailableforsaleSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentSecondaryCategorizationAxis=pfg_TradingSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CollateralizedDebtObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_OtherDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=pfg_OtherLimitedPartnershipAndFundInterestsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: