Segment Information (Details 4) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Operating Revenues for Products and Services
Operating revenues	$ 2,136.3	$ 2,098.5	$ 4,243.7	$ 4,146.0
Net realized capital gains (losses), net of related revenue adjustments	(21.7)	12.2	(52.1)	(68.3)
Exited group medical insurance business	4.0	180.8	22.9	435.7
Assumption change within Individual Life business		4.9		4.9
Total revenues	2,118.6	2,296.4	4,214.5	4,518.3
Retirement and Investor Services
Operating Revenues for Products and Services
Operating revenues	1,081.2	1,044.2	2,136.3	2,062.1
Retirement and Investor Services \| Total Accumulation
Operating Revenues for Products and Services
Operating revenues	769.4	770.8	1,516.8	1,524.9
Retirement and Investor Services \| Full service accumulation
Operating Revenues for Products and Services
Operating revenues	334.4	342.3	667.1	685.7
Retirement and Investor Services \| Principal Funds
Operating Revenues for Products and Services
Operating revenues	149.1	144.2	296.2	285.8
Retirement and Investor Services \| Individual annuities
Operating Revenues for Products and Services
Operating revenues	289.6	288.2	561.9	562.4
Retirement and Investor Services \| Bank and trust services
Operating Revenues for Products and Services
Operating revenues	25.3	24.6	49.8	48.4
Retirement and Investor Services \| Accumulation Eliminations
Operating Revenues for Products and Services
Operating revenues	(29.0)	(28.5)	(58.2)	(57.4)
Retirement and Investor Services \| Total Guaranteed
Operating Revenues for Products and Services
Operating revenues	311.8	273.4	619.5	537.2
Retirement and Investor Services \| Investment only
Operating Revenues for Products and Services
Operating revenues	108.3	127.8	223.6	263.4
Retirement and Investor Services \| Full service payout
Operating Revenues for Products and Services
Operating revenues	203.5	145.6	395.9	273.8
Principal Global Investors
Operating Revenues for Products and Services
Operating revenues	141.1	136.3	279.2	261.6
Inter-segment revenues	53.3	55.8	105.9	107.5
Principal International
Operating Revenues for Products and Services
Operating revenues	210.6	227.2	473.1	433.3
U.S. Insurance Solutions
Operating Revenues for Products and Services
Operating revenues	751.5	730.7	1,448.5	1,462.7
U.S. Insurance Solutions \| Individual life insurance
Operating Revenues for Products and Services
Operating revenues	358.7	353.5	672.2	711.8
U.S. Insurance Solutions \| Specialty benefits insurance
Operating Revenues for Products and Services
Operating revenues	392.8	377.2	776.3	750.9
Corporate
Operating Revenues for Products and Services
Operating revenues	$ (48.1)	$ (39.9)	$ (93.4)	$ (73.7)

Aggregate revenue recognized during the period for an assumption change in the Individual Life business.

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- Definition

Aggregate revenue recognized during the period for an exited group medical insurance business that does not qualify for discontinued accounting treatment.

+ References

No definition available.

+ Details

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- Definition

Net realized capital gains (losses) net of recognition of deferred front-end fee revenues for sales charges on retirement products and services and certain market value adjustments to fee revenues. It excludes periodic settlements and accruals on derivatives not designated as hedging instruments.

+ References

No definition available.

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- Definition

Aggregate revenue recognized during the period (derived from goods sold, services rendered, insurance premiums, or other activities that constitute an entity's earning process). For financial services companies, also includes investment and interest income, and sales and trading gains.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1)

-URI http://asc.fasb.org/extlink&oid=6880815&loc=d3e20235-122688

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Article 5

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- Definition

Amount of revenue from transactions with other operating segments of the same entity.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 131

-Paragraph 27

-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.

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- Definition

Amount of total revenue for the reportable segments, including revenue from external customers, transactions with other operating segments of the same entity, and (gross) interest revenue. All considered to be "operating revenue" for purposes of the taxonomy.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 280

-SubTopic 10

-Section 50

-Paragraph 22

-Subparagraph (a),(b),(c)

-URI http://asc.fasb.org/extlink&oid=6534315&loc=d3e8736-108599

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 131

-Paragraph 27

-Subparagraph a, b, 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.

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