Segment Information (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2014	Jun. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information
Net realized capital gains (losses), net of related revenue adjustments	$ (68.2)		$ (76.7)	$ (25.0)	$ (253.4)
Total revenues	2,484.3		2,239.6	7,569.6	6,616.9
Net realized capital gains (losses), as adjusted	(55.2)		(22.8)	(47.3)	(126.8)
Net income (loss) available to common stockholders	240.7		245.7	840.7	646.3
Impact of court ruling on uncertain tax positions		(47.5)
Operating Segments
Segment Reporting Information
Operating revenues	2,552.7		2,315.8	7,594.6	6,865.8
Operating earnings (loss)	353.7		269.2	993.9	773.9
Operating Segments \| Retirement and Investor Services
Segment Reporting Information
Operating revenues	1,324.7		1,136.5	3,851.0	3,387.6
Operating earnings (loss)	204.3		172.9	632.9	515.2
Operating Segments \| Principal Global Investors
Segment Reporting Information
Operating revenues	173.6		160.9	517.9	482.8
Operating earnings (loss)	25.3		23.1	79.6	72.4
Operating Segments \| Principal International
Segment Reporting Information
Operating revenues	294.5		303.3	952.2	825.9
Operating earnings (loss)	73.8		50.7	205.1	153.6
Operating Segments \| U.S. Insurance Solutions
Segment Reporting Information
Operating revenues	821.0		767.6	2,436.8	2,318.6
Operating earnings (loss)	83.3		54.0	175.7	136.9
Operating Segments \| Corporate
Segment Reporting Information
Operating revenues	(61.1)		(52.5)	(163.3)	(149.1)
Operating earnings (loss)	(33.0)		(31.5)	(99.4)	(104.2)
Reconciling Items
Segment Reporting Information
Net realized capital gains (losses), net of related revenue adjustments	(68.2)		(76.7)	(25.0)	(253.4)
Exited group medical insurance business	(0.2)		0.5		4.5
Net realized capital gains (losses), as adjusted	(55.2)		(22.8)	(47.3)	(126.8)
Other after-tax adjustments	(57.8)		(0.7)	(105.9)	(0.8)
Impact of enactment of tax legislation	(58.1)			(58.1)
Impact of court ruling on uncertain tax positions				(47.5)
After-tax gains (losses) associated with exited group medical insurance business that does not qualify for discontinued operations	$ 0.3		$ (0.7)	$ (0.3)	$ (0.8)

Other after-tax adjustment for the impact of a court ruling on some uncertain tax positions.

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Other after-tax adjustment for the change in net deferred tax liabilities resulting from enactment of tax legislation.

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

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The after-tax gain (loss) resulting from an exited group medical insurance business that is not a discontinued operation. It is included in income from continuing operations in the income statement.

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- Definition

Operating earnings is a measure of profit or loss for the operating segments that is used to evaluate performance on a basis comparable to that used by securities analysts. It is derived by adjusting net income for net realized capital gains (losses), as adjusted, and other after-tax adjustments management believes are not indicative of overall operating trends.

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Operating revenues is a measure of revenue for the operating segments. It is derived by adjusting total revenues for net realized capital gains and losses (except periodic settlements and accruals on derivatives not designated as hedging instruments), including their impact on recognition of front-end fee revenues, certain market value adjustments to fee revenues and amortization of hedge accounting book value adjustments for certain discontinued hedges, and revenue from the exited group medical insurance business. Operating revenues also includes operating revenues from real estate properties that qualify for discontinued operations.

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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.

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Net realized capital gains (losses) net of income taxes, related changes in the amortization pattern of deferred acquisition costs and sales inducements, recognition of deferred front-end fee revenues for sales charges on retirement products and services, net realized capital gains and losses distributed, noncontrolling interest capital gains and losses and certain market value adjustments to fee revenues. Net realized capital gains (losses), as adjusted, exclude periodic settlements and accruals on derivative instruments not designated as hedging instruments and exclude certain market value adjustments of embedded derivatives.

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The after-tax impact of specific transactions that are not recurring in nature and that management does not believe are indicative of overall operating trends.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 260

-SubTopic 10

-Section 45

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e1377-109256

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

+ 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=26872669&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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