Derivative Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Dec. 31, 2012
Derivative Financial Instruments, exposure
GMWB minimum account balance to make withdrawals by customer	$ 0
Cash and securities posted under collateral arrangements associated with derivative credit support agreements	382.9	296.9
Aggregate fair value of all derivative instruments with credit-risk-related contingent features that were in a liability position	1,199.4	1,205.4
Collateral posted supporting derivatives with credit-risk-related contingent features that were in a liability position	382.9	296.9
Additional collateral required to be posted if derivative credit-risk-related contingent features were triggered	71.2
Cash collateral received associated with derivative credit support annex agreements	73.7	207.8
Notional amount	33,944.4	34,256.4
Gross credit exposure	893.0	1,086.7
Less: collateral received	131.4	248.0
Net credit exposure	761.6	838.7
Interest rate swaps
Derivative Financial Instruments, exposure
Cash exchanged under contract	0
Principal payments made under contract	0
Notional amount	18,897.8	18,381.2
Gross credit exposure	612.1	683.9
Interest rate options
Derivative Financial Instruments, exposure
Notional amount	1,500.0	500.0
Gross credit exposure	46.3	48.5
Swaptions
Derivative Financial Instruments, exposure
Notional amount	325.0	325.0
Gross credit exposure	0.8	0.7
Interest rate futures
Derivative Financial Instruments, exposure
Notional amount	187.0	82.0
Foreign currency swaps
Derivative Financial Instruments, exposure
Notional amount	3,287.8	3,454.1
Gross credit exposure	168.4	263.8
Currency forwards
Derivative Financial Instruments, exposure
Notional amount	251.3	557.2
Gross credit exposure	6.6	6.8
Foreign currency options
Derivative Financial Instruments, exposure
Notional amount		1,400.0
Gross credit exposure		1.9
Equity options
Derivative Financial Instruments, exposure
Notional amount	1,827.2	1,811.8
Gross credit exposure	50.8	74.3
Equity futures
Derivative Financial Instruments, exposure
Notional amount	363.6	373.6
Credit default swaps
Derivative Financial Instruments, exposure
Notional amount	1,254.2	1,378.3
Gross credit exposure	7.7	6.8
Total return swaps
Derivative Financial Instruments, exposure
Notional amount	100.0	100.0
Gross credit exposure	0.3
Embedded derivative financial instruments
Derivative Financial Instruments, exposure
Notional amount	$ 5,950.5	$ 5,893.2

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 12

-Section 13

-Sentence Column B

+ Details

Name:	invest_DerivativeNotionalAmount
Namespace Prefix:	invest_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the minimum account balance to make withdrawals by customer under the variable annuities with guaranteed minimum withdrawal benefits.

+ References

No definition available.

+ Details

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

- Definition

The carrying value as of the balance sheet date of the cash received as collateral associated with derivative credit support annex agreements.

+ References

No definition available.

+ Details

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

- Definition

The carrying value as of the balance sheet date of the cash and securities pledged as collateral associated with derivative credit support agreements.

+ References

No definition available.

+ Details

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

- Definition

The carrying value as of the balance sheet date of the cash and securities received as collateral associated with derivative credit support agreements.

+ References

No definition available.

+ Details

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

- Definition

The amount at risk if a counterparty to the derivative instruments does not perform.

+ References

No definition available.

+ Details

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

- Definition

The amount at risk if a counterparty to the derivative instrument does not perform, after considering collateral received.

+ References

No definition available.

+ Details

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

- Definition

Represents the amount of cash exchanged under the interest rate derivative contracts.

+ References

No definition available.

+ Details

Name:	pfg_InterestRateDerivativesCashExchanged
Namespace Prefix:	pfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Represents the amount of principal payments made by parties to the interest rate derivative contracts.

+ References

No definition available.

+ Details

Name:	pfg_InterestRateDerivativesPrincipalPayments
Namespace Prefix:	pfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624258-113959

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44D

-Subparagraph c

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

- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44D

-Subparagraph c

-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 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624258-113959

+ Details

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

- Details

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

- Definition

The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4H

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5624258-113959

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 44D

-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.

+ Details

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