Fair Value Measurements (Details 2) (USD $) In Millions, unless otherwise specified	Sep. 30, 2011	Dec. 31, 2010
Fixed maturities valued using internal pricing models
Fixed maturities classified as Level 3 assets, percent valued using internal pricing models (as a percent)	1.00%
Assets (liabilities) measured at fair value on a recurring basis
Derivative assets	$ 1,350.0	$ 1,083.2
Separate account assets	67,145.8	69,555.3
Investment-type insurance contracts	(166.1)	(6.6)
Fixed maturities \| Recurring Fair Value Measurements \| Assets (liabilities) measured at fair value
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	49,069.5	48,636.3
Trading	1,278.6	1,120.3
Fixed maturities \| Recurring Fair Value Measurements \| Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	134.7	325.0
Trading	343.9	159.8
Fixed maturities \| Recurring Fair Value Measurements \| Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	48,525.9	47,520.4
Trading	717.6	691.4
Fixed maturities \| Recurring Fair Value Measurements \| Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	408.9	790.9
Trading	217.1	269.1
Corporate debt securities \| Recurring Fair Value Measurements \| Assets (liabilities) measured at fair value
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	33,844.0	33,892.5
Corporate debt securities \| Recurring Fair Value Measurements \| Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	87.6	95.4
Corporate debt securities \| Recurring Fair Value Measurements \| Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	33,497.5	33,245.0
Corporate debt securities \| Recurring Fair Value Measurements \| Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	258.9	552.1
U.S. government and agencies \| Recurring Fair Value Measurements \| Assets (liabilities) measured at fair value
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	792.7	769.3
U.S. government and agencies \| Recurring Fair Value Measurements \| Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	47.1	229.6
U.S. government and agencies \| Recurring Fair Value Measurements \| Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	745.6	539.7
Non-U.S. governments \| Recurring Fair Value Measurements \| Assets (liabilities) measured at fair value
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	791.7	872.6
Non-U.S. governments \| Recurring Fair Value Measurements \| Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	769.0	848.1
Non-U.S. governments \| Recurring Fair Value Measurements \| Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	22.7	24.5
Residential mortgage-backed pass-through securities \| Recurring Fair Value Measurements \| Assets (liabilities) measured at fair value
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	3,278.3	3,196.2
Residential mortgage-backed pass-through securities \| Recurring Fair Value Measurements \| Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	3,278.3	3,196.2
Commercial mortgage-backed securities \| Recurring Fair Value Measurements \| Assets (liabilities) measured at fair value
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	3,575.3	3,842.2
Commercial mortgage-backed securities \| Recurring Fair Value Measurements \| Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	3,575.3	3,826.0
Commercial mortgage-backed securities \| Recurring Fair Value Measurements \| Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale		16.2
States and political subdivisions \| Recurring Fair Value Measurements \| Assets (liabilities) measured at fair value
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	2,937.3	2,656.4
States and political subdivisions \| Recurring Fair Value Measurements \| Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	2,937.3	2,656.4
Collateralized debt obligations \| Recurring Fair Value Measurements \| Assets (liabilities) measured at fair value
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	332.2	293.0
Collateralized debt obligations \| Recurring Fair Value Measurements \| Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	233.3	183.7
Collateralized debt obligations \| Recurring Fair Value Measurements \| Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	98.9	109.3
Other debt obligations \| Recurring Fair Value Measurements \| Assets (liabilities) measured at fair value
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	3,518.0	3,114.1
Other debt obligations \| Recurring Fair Value Measurements \| Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	3,489.6	3,025.3
Other debt obligations \| Recurring Fair Value Measurements \| Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	28.4	88.8
Equity securities \| Recurring Fair Value Measurements \| Assets (liabilities) measured at fair value
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	120.5	169.9
Trading	360.3	316.9
Equity securities \| Recurring Fair Value Measurements \| Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	111.8	124.1
Trading	250.1	212.9
Equity securities \| Recurring Fair Value Measurements \| Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	2.6	2.6
Trading	110.2	104.0
Equity securities \| Recurring Fair Value Measurements \| Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	6.1	43.2
Recurring Fair Value Measurements \| Assets (liabilities) measured at fair value
Assets (liabilities) measured at fair value on a recurring basis
Derivative assets	1,350.0	1,083.2
Other investments	224.0	210.7
Cash equivalents	519.3	1,247.2
Sub-total excluding separate account assets	52,922.2	52,784.5
Separate account assets	67,145.8	69,555.3
Total assets	120,068.0	122,339.8
Investment-type insurance contracts	(166.1)	(6.6)
Derivative liabilities	(1,587.5)	(1,274.5)
Other liabilities	(226.9)	(250.3)
Total liabilities	(1,980.5)	(1,531.4)
Net assets (liabilities)	118,087.5	120,808.4
Recurring Fair Value Measurements \| Fair value hierarchy Level 1
Assets (liabilities) measured at fair value on a recurring basis
Other investments	13.0	14.1
Cash equivalents	157.3	217.3
Sub-total excluding separate account assets	1,010.8	1,053.2
Separate account assets	42,942.0	51,012.9
Total assets	43,952.8	52,066.1
Net assets (liabilities)	43,952.8	52,066.1
Recurring Fair Value Measurements \| Fair value hierarchy Level 2
Assets (liabilities) measured at fair value on a recurring basis
Derivative assets	1,283.9	1,049.9
Other investments	106.5	68.3
Cash equivalents	362.0	1,029.9
Sub-total excluding separate account assets	51,108.7	50,466.5
Separate account assets	20,146.8	14,770.9
Total assets	71,255.5	65,237.4
Derivative liabilities	(1,383.3)	(1,093.0)
Other liabilities	(70.9)	(93.5)
Total liabilities	(1,454.2)	(1,186.5)
Net assets (liabilities)	69,801.3	64,050.9
Recurring Fair Value Measurements \| Fair value hierarchy Level 3
Assets (liabilities) measured at fair value on a recurring basis
Derivative assets	66.1	33.3
Other investments	104.5	128.3
Sub-total excluding separate account assets	802.7	1,264.8
Separate account assets	4,057.0	3,771.5
Total assets	4,859.7	5,036.3
Investment-type insurance contracts	(166.1)	(6.6)
Derivative liabilities	(204.2)	(181.5)
Other liabilities	(156.0)	(156.8)
Total liabilities	(526.3)	(344.9)
Net assets (liabilities)	4,333.4	4,691.4
Fixed maturities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	49,069.5	48,636.3
U.S. government and agencies
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	792.7	769.3
Non-U.S. governments
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	791.7	872.6
States and political subdivisions
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	2,937.3	2,656.4
Corporate debt securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	33,844.0	33,892.5
Residential mortgage-backed pass-through securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	3,278.3	3,196.2
Commercial mortgage-backed securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	3,575.3	3,842.2
Collateralized debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	332.2	293.0
Other debt obligations
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	3,518.0	3,114.1
Equity securities
Assets (liabilities) measured at fair value on a recurring basis
Available-for-sale	$ 120.5	$ 169.9

This item represents the fair value of embedded derivatives that have been bifurcated from an investment-type insurance host contract.

Name:	pfg_DerivativeFairValueOfDerivativeLiabilityInvestmentTypeInsuranceContracts
Namespace Prefix:	pfg
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This element represents the net of total assets and liabilities reported on the balance sheet at period end measured at fair value by the entity.

+ Details

Name:	pfg_FairValueAssetsAndLiabiltiesMeasuredOnRecurringBasisNetAssetsLiabilties
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 measured on a recurring basis associated with domain members defined in one or many axes to the table.

+ Details

Name:	pfg_FairValueAssetsLiabilitiesMeasuredOnRecurringBasisFinancialStatementCaptionsLineItems
Namespace Prefix:	pfg
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This item represents short-term invested assets with a maturity date of three months or less when purchased that are required to be reported at fair value in the statement of financial position.

+ Details

Name:	pfg_FairValueAssetsMeasuredOnRecurringBasisCashEquivalents
Namespace Prefix:	pfg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ Details

Name:	pfg_FairValueAssetsMeasuredOnRecurringBasisOtherInvestments
Namespace Prefix:	pfg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents assets, other than separate account assets, measured at fair value on a recurring basis.

+ Details

Name:	pfg_FairValueAssetsMeasuredOnRecurringBasisTotalAssetsExcludingSeparateAccountAssets
Namespace Prefix:	pfg
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The percentage of fixed maturity securities classified as Level 3 that were valued using internal pricing models.

+ Details

Name:	pfg_FixedMaturitySecuritiesLevel3ValuedUsingInternalPricingModels
Namespace Prefix:	pfg
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	pfg_FixedMaturitySecuritiesValuedUsingInternalPricingModelsAbstract
Namespace Prefix:	pfg
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This item represents certain embedded derivatives to deliver underlying securities of structured investments to third parties and obligations of consolidated variable interest entities reported at fair value.

+ Details

Name:	pfg_LiabilitiesOtherFairValueDisclosure
Namespace Prefix:	pfg
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

+ Details

Name:	us-gaap_AssetsFairValueDisclosure
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: 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=6379932&loc=d3e22054-111558

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesFairValueDisclosure
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4B

-Subparagraph (a),(c)

-URI http://asc.fasb.org/extlink&oid=6935481&loc=SL5624163-113959

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44C

-Subparagraph a

-Clause 1

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 205G

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

Name:	us-gaap_DerivativeFairValueOfDerivativeAsset
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Derivative instrument obligations meeting the definition of a liability which the Company is a party to as of the balance sheet date.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 10

-Subparagraph i

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 6

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 15

-Paragraph 83

-URI http://asc.fasb.org/extlink&oid=6913464&loc=d3e34841-113949

+ Details

Name:	us-gaap_DerivativeFinancialInstrumentsLiabilitiesFairValueDisclosure
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This element represents the aggregate of the liabilities reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.

+ Details

Name:	us-gaap_LiabilitiesFairValueDisclosure
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

-Article 7

-Section 03

-Paragraph 11

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 SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 10

-Article 9

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Separate Account

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

-Topic 944

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

-Publisher AICPA

-Name Statement of Position (SOP)

-Number 03-1

-Paragraph 10-18

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 113

-Paragraph 53, 54

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Trading Securities which consist of all investments in certain debt and equity securities that are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) or for which an entity has elected the fair value option. Trading generally reflects active and frequent buying and selling, and trading securities are generally used with the objective of generating profits on short-term differences in price, but this category of investments may also include items which are not intended to be sold in the near term or for which the fair value option has been elected.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6379932&loc=d3e22054-111558

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

+ Details

Name:	us-gaap_TradingSecuritiesFairValueDisclosure
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant