Fair Value Measurements - Financial Instruments Not Reported at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets (liabilities)
Mortgage loans	$ 14,150.5	$ 13,230.2
Policy loans	808.3	823.8
Short-term debt	(39.5)	(51.4)
Long-term debt	(3,178.4)	(3,125.7)
Carrying amount
Assets (liabilities)
Mortgage loans	14,150.5	13,230.2
Policy loans	808.3	823.8
Other investments	236.8	230.3
Cash and cash equivalents not required to be reported at fair value	677.5	772.5
Investment contracts	(31,003.0)	(31,089.4)
Short-term debt	(39.5)	(51.4)
Long-term debt	(3,178.4)	(3,125.7)
Separate account liabilities	(145,552.5)	(127,452.1)
Bank deposits	(2,336.4)	(2,199.8)
Cash collateral payable	(125.8)	(575.7)
Assets (liabilities) measured at fair value
Assets (liabilities)
Mortgage loans	14,443.2	13,453.2
Policy loans	998.4	1,011.0
Other investments	234.0	236.8
Cash and cash equivalents not required to be reported at fair value	677.5	772.5
Investment contracts	(30,468.8)	(30,622.6)
Short-term debt	(39.5)	(51.4)
Long-term debt	(3,442.5)	(3,242.0)
Separate account liabilities	(144,300.5)	(126,282.0)
Bank deposits	(2,328.9)	(2,204.1)
Cash collateral payable	(125.8)	(575.7)
Assets (liabilities) measured at fair value \| Fair value hierarchy Level 1
Assets (liabilities)
Cash and cash equivalents not required to be reported at fair value	659.9	731.4
Bank deposits	(1,780.3)	(1,585.1)
Cash collateral payable	(125.8)	(575.7)
Assets (liabilities) measured at fair value \| Fair value hierarchy Level 2
Assets (liabilities)
Other investments	159.4	157.7
Cash and cash equivalents not required to be reported at fair value	17.6	41.1
Investment contracts	(4,736.1)	(5,400.8)
Short-term debt	(39.5)	(51.4)
Long-term debt	(3,393.5)	(3,242.0)
Bank deposits	(548.6)	(619.0)
Assets (liabilities) measured at fair value \| Fair value hierarchy Level 3
Assets (liabilities)
Mortgage loans	14,443.2	13,453.2
Policy loans	998.4	1,011.0
Other investments	74.6	79.1
Investment contracts	(25,732.7)	(25,221.8)
Long-term debt	(49.0)
Separate account liabilities	$ (144,300.5)	$ (126,282.0)

Includes currency on hand and other cash equivalents not required to be reported at fair value.

No definition available.

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

- Definition

This item represents the obligation to return cash received as collateral from counterparties.

+ References

No definition available.

+ Details

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

- Definition

This item represents investment contracts such as guaranteed investment contracts and funding arrangements that do not have significant insurance risk. This item excludes any embedded derivatives within investment host contracts that are required to be reported at fair value.

+ References

No definition available.

+ Details

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

- Definition

This item represents invested assets not specifically identified in the statement of financial position, excluding derivative assets. This includes invested assets such as seed money and commercial mortgage loans of consolidated variable interest entities reported at fair value.

+ References

No definition available.

+ Details

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

- Definition

This item represents investment-type separate account liabilities that do not have significant insurance risk.

+ References

No definition available.

+ Details

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

- Definition

The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.12)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount before allowance of loans made to policy and contract holders using their life insurance contracts' cash value or separate accounts as collateral. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums over and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

+ Details

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.

+ References

+ Details

Name:	us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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://www.xbrl.org/2003/role/presentationRef
-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=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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=6876686&loc=d3e534808-122878

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-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=6879938&loc=d3e572229-122910

+ Details

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

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: