Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	$ 49,755.7	$ 49,670.8
Fixed maturities, trading	574.0us-gaap_TradingSecuritiesDebt	604.6us-gaap_TradingSecuritiesDebt
Equity securities, trading	875.1us-gaap_TradingSecuritiesEquity	840.2us-gaap_TradingSecuritiesEquity
Real estate	1,391.7us-gaap_RealEstateInvestments	1,344.6us-gaap_RealEstateInvestments
Other investments	3,395.6pfg_OtherInvestmentsAsDefined	3,209.8pfg_OtherInvestmentsAsDefined
Accrued investment income	531.1	505.9
Separate account assets	143,505.2us-gaap_SeparateAccountAssets	140,072.8us-gaap_SeparateAccountAssets
Other assets	1,013.7us-gaap_OtherAssets	1,083.5us-gaap_OtherAssets
Total assets	222,494.9us-gaap_Assets	219,087.0us-gaap_Assets
Long-term debt	2,514.6us-gaap_LongTermDebt	2,531.2us-gaap_LongTermDebt
Income taxes currently payable	12.0us-gaap_AccruedIncomeTaxes	11.5us-gaap_AccruedIncomeTaxes
Deferred income taxes	968.5us-gaap_DeferredTaxLiabilities	1,035.3us-gaap_DeferredTaxLiabilities
Separate account liabilities	143,505.2us-gaap_SeparateAccountsLiability	140,072.8us-gaap_SeparateAccountsLiability
Other liabilities	5,506.3us-gaap_OtherLiabilities	5,542.2us-gaap_OtherLiabilities
Total liabilities	211,937.8us-gaap_Liabilities	208,797.0us-gaap_Liabilities
Aggregate consolidated variable interest entities
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	266.4	278.2
Fixed maturities, trading	100.4	100.4
Equity securities, trading	352.3	345.3
Real estate	297.8	284.9
Other investments	34.6	40.6
Cash	6.0	4.7
Accrued investment income	2.3	2.0
Separate account assets	35,319.4	34,655.4
Other assets	0.3	0.3
Total assets	36,379.5	35,711.8
Long-term debt	65.6	82.3
Income taxes currently payable	0.2	10.6
Deferred income taxes	1.3	1.1
Separate account liabilities	35,319.4	34,655.4
Other liabilities	335.9	344.0
Total liabilities	35,722.4	35,093.4
Grantor trusts
Consolidated Variable Interest Entity disclosures
Number of consolidated variable interest entities	3
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, available-for-sale	266.4	278.2
Accrued investment income	0.4	0.4
Total assets	266.8	278.6
Deferred income taxes	1.7	1.5
Other liabilities	233.8	239.1
Total liabilities	235.5	240.6
Collateralized private investment vehicle
Consolidated Variable Interest Entity disclosures
Number of entities for which the reporting entity acts as primary beneficiary	1
Carrying amounts of consolidated VIE assets and liabilities
Fixed maturities, trading	100.4	100.4
Total assets	100.4	100.4
Other liabilities	86.7	85.6
Total liabilities	86.7	85.6
Commercial mortgage-backed securities VIE
Carrying amounts of consolidated VIE assets and liabilities
Other investments	28.9	35.0
Accrued investment income	0.2	0.2
Total assets	29.1	35.2
Other liabilities	1.9	4.8
Total liabilities	1.9	4.8
Mandatory retirement savings
Carrying amounts of consolidated VIE assets and liabilities
Equity securities, trading	352.3	345.3
Separate account assets	35,319.4	34,655.4
Total assets	35,671.7	35,000.7
Separate account liabilities	35,319.4	34,655.4
Total liabilities	35,319.4	34,655.4
Real estate VIE
Carrying amounts of consolidated VIE assets and liabilities
Real estate	297.8	284.9
Other investments	5.7	5.6
Cash	6.0	4.7
Accrued investment income	1.7	1.4
Other assets	0.3	0.3
Total assets	311.5	296.9
Long-term debt	65.6	82.3
Income taxes currently payable	0.2	10.6
Deferred income taxes	(0.4)	(0.4)
Other liabilities	13.5	14.5
Total liabilities	$ 78.9	$ 107.0

Represents the number of entities for which the reporting entity acts as primary beneficiary.

No definition available.

Name:	pfg_NumberOfEntitiesForWhichReportingEntityActsAsPrimaryBeneficiary
Namespace Prefix:	pfg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Other investments include, but are not limited to, derivative assets, noncontrolling interests in subsidiaries and real estate, commercial mortgage loans of consolidated variable interest entities and company's ownership in corporate sponsored mutual funds.

+ References

No definition available.

+ Details

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

- Definition

Represents the number of variable interest entities consolidated by the entity as the primary beneficiary of the VIEs.

+ References

No definition available.

+ Details

Name:	pfg_VariableInterestEntitiesConsolidatedNumber
Namespace Prefix:	pfg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of the unpaid sum of the known and estimated amounts payable to satisfy all domestic and foreign income tax obligations due. This amount is the total of current and noncurrent accrued income taxes.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 15

-Subparagraph b(1)

-Article 7

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319

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 210.7-03.15(b))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.15)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 15

-Subparagraph 1

-Article 9

+ Details

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

- Definition

Interest, dividends, rents, ancillary and other revenues earned but not yet received by the entity on its investments.

+ References

+ Details

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

- Definition

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

+ Details

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

- Definition

Amount of debt securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 320

-Section 50

-Paragraph 2

-Subparagraph (b)-(g)

-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803

+ Details

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

- Definition

Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Excludes cash and cash equivalents within disposal group and discontinued operation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash

-URI http://asc.fasb.org/extlink&oid=6506951

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 1

-Article 5

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 210.5-02.1)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Amount of deferred tax liability attributable to taxable temporary differences net of deferred tax asset attributable to deductible temporary differences and carryforwards after valuation allowances.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e31931-109318

+ Details

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

- Definition

Sum of the carrying amounts as of the balance sheet date of all liabilities that are recognized. Liabilities are probable future sacrifices of economic benefits arising from present obligations of an entity to transfer assets or provide services to other entities in the future.

+ References

+ Details

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

- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. 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 SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 7

Reference 2: 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

Reference 3: 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 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 20, 22

-Article 5

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Definition

The aggregate carrying amounts, as of the balance sheet date, of assets not separately disclosed in the balance sheet.

+ References

+ Details

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

- Definition

The aggregate carrying amount, as of the balance sheet date, of liabilities not separately disclosed in the balance sheet.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 15

-Article 9

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 210.9-03.15)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Definition

Amount of real estate investments, net of accumulated depreciation, which may include the following: (1) land available-for-sale; (2) land available-for-development; (3) investments in building and building improvements; (4) tenant allowances; (5) developments in-process; (6) rental properties; (7) other real estate investments; (8) real estate joint ventures; and (9) unconsolidated real estate and other joint ventures not separately presented.

+ 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 210.7-03.1(d))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph d

-Article 7

+ Details

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

- Definition

The fair value of the assets held by the Entity for the benefit of separate account policyholders.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 10

-Article 9

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 80

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6484091&loc=d3e19268-158472

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-Glossary Separate Account

-URI http://asc.fasb.org/extlink&oid=6524887

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 11

-Article 7

+ Details

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

- Definition

Amount of liability for variable contracts that meet specified criteria that are backed by assets held in separate accounts and carried at the fair value of those assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 80

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6484091&loc=d3e19268-158472

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-Glossary Separate Account

-URI http://asc.fasb.org/extlink&oid=6524887

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 80

-Section 50

-Paragraph 1

-Subparagraph (b)(1)

-URI http://asc.fasb.org/extlink&oid=6484115&loc=d3e19393-158473

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 18

-Article 7

+ Details

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

- Definition

Amount of investments in debt securities classified as trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.4)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: 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),1)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Definition

Fair value of investments in trading equity securities and other forms of trading securities that provide ownership interests.

+ 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 210.5-02.2)

-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 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.4)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 6: 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),1(b))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=pfg_VariableInterestEntityPrimaryBeneficiaryCollateralizedPrivateInvestmentVehiclesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=pfg_VariableInterestEntityPrimaryBeneficiaryCommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=pfg_VariableInterestEntityPrimaryBeneficiaryGrantorTrustsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=pfg_VariableInterestEntityPrimaryBeneficiaryMandatoryRetirementSavingsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=pfg_VariableInterestEntityPrimaryBeneficiaryRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: