Segment Information (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Segment Information: Assets
Total assets	$ 143,302.3	$ 145,631.1
Retirement and Investor Services
Segment Information: Assets
Total assets	105,200.8	110,043.0
Principal Global Investors
Segment Information: Assets
Total assets	1,615.3	1,308.1
Principal International
Segment Information: Assets
Total assets	15,366.4	12,774.5
U.S. Insurance Solutions
Segment Information: Assets
Total assets	17,318.1	16,558.2
Corporate
Segment Information: Assets
Total assets	$ 3,801.7	$ 4,947.3

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.

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 7

-Section 03

-Paragraph 12

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

-Topic 210

-SubTopic 10

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

-Section S99

-Name Accounting Standards Codification

-Publisher FASB

-Paragraph 1

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

-Publisher FASB

-Name Statement of Financial Accounting Concepts (CON)

-Number 6

-Paragraph 25

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 18

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.18)

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

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