Fair Value Measurements (Details 4) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Fair Value Hierarchy Levels Transfers
Transfers into Level 3, assets	$ 159.2	$ 226.1	$ 531.9
Transfers out of Level 3, assets	454.8	497.1	414.1
Transfers out of Level 3, liabilities	136.3
Separate account assets transferred out of Level 1 into Level 2	3,595.9	3,128.3
Separate account assets transferred out of Level 2 into Level 1	$ 2,796.1	$ 6,600.6

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This element represents significant transfers of separate account assets into Level 2 from Level 1 of the fair value hierarchy.

+ References

No definition available.

+ Details

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- Definition

This element represents significant transfers of separate account assets into Level 1 from Level 2 of the fair value hierarchy.

+ References

No definition available.

+ Details

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Namespace Prefix:	pfg_
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Period Type:	duration

- Definition

Transfers out of liabilities measured at fair value and categorized within Level 3 of the fair value hierarchy that have taken place during the period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(3)

-URI http://asc.fasb.org/extlink&oid=6925170&loc=d3e19207-110258

+ Details

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- Definition

Transfers into assets measured at fair value and categorized within Level 3 of the fair value hierarchy that have taken place during the period.

+ References

+ Details

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- Definition

Transfers out of assets measured at fair value and categorized within Level 3 of the fair value hierarchy that have taken place during the period.

+ References

+ Details

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