Segment Information - Reconciliation of Segment Operating Revenues and Earnings to Consolidated (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Operating Revenue And Profit (Loss) From Segments To Consolidated
Net realized capital gains (losses), net of related revenue adjustments	$ 7.5	$ (141.3)	$ 121.1	$ (95.7)
Total revenues	3,025.7	3,259.2	6,062.3	5,916.5
Pre-tax net realized capital gains (losses), as adjusted	(16.0)	(128.0)	80.8	(94.5)
Income (loss) before income taxes	399.9	294.5	839.7	752.6
Impact of court ruling on uncertain tax positions				15.1
Pre-tax gains (losses) associated with exited group medical insurance business that does not qualify for discontinued operations				(0.3)
Retirement and Income Solutions
Operating Revenue And Profit (Loss) From Segments To Consolidated
Inter-segment revenues	92.6	112.8	181.8	218.0
Principal Global Investors
Operating Revenue And Profit (Loss) From Segments To Consolidated
Inter-segment revenues	57.0	53.5	111.3	113.9
Operating Segments
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	3,035.1	3,420.1	5,973.0	5,979.3
Pre-tax operating earnings (losses)	431.2	429.5	787.5	850.4
Operating Segments \| Retirement and Income Solutions
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	1,474.6	1,905.2	2,938.6	3,029.7
Pre-tax operating earnings (losses)	194.7	216.9	376.1	419.8
Operating Segments \| Retirement and Income Solutions \| Retirement and Income Solutions - Fee
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	428.8	454.3	842.1	892.0
Operating Segments \| Retirement and Income Solutions \| Retirement and Income Solutions - Spread
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	1,045.8	1,450.9	2,096.5	2,137.7
Operating Segments \| Principal Global Investors
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	351.3	337.8	660.8	666.7
Pre-tax operating earnings (losses)	117.5	98.5	197.2	191.1
Operating Segments \| Principal International
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	318.7	328.3	605.3	579.1
Pre-tax operating earnings (losses)	69.9	72.2	137.9	153.0
Operating Segments \| U.S. Insurance Solutions
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	908.0	857.8	1,798.9	1,719.5
Pre-tax operating earnings (losses)	103.6	88.1	184.1	170.0
Operating Segments \| U.S. Insurance Solutions \| Specialty benefits insurance
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	498.2	464.7	981.1	931.0
Operating Segments \| U.S. Insurance Solutions \| Individual life insurance
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	409.9	393.2	817.9	788.6
Operating Segments \| U.S. Insurance Solutions \| U.S. Insurance Solutions Eliminations
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	(0.1)	(0.1)	(0.1)	(0.1)
Operating Segments \| Corporate
Operating Revenue And Profit (Loss) From Segments To Consolidated
Operating revenues	(17.5)	(9.0)	(30.6)	(15.7)
Pre-tax operating earnings (losses)	(54.5)	(46.2)	(107.8)	(83.5)
Reconciling Items
Operating Revenue And Profit (Loss) From Segments To Consolidated
Net realized capital gains (losses), net of related revenue adjustments	7.5	(141.3)	121.1	(95.7)
Certain revenue adjustments related to equity method investments	(16.9)	(13.3)	(31.8)	(27.9)
Other income on a tax indemnification		(6.7)		60.2
Exited group medical insurance business		0.4		0.6
Pre-tax net realized capital gains (losses), as adjusted	(16.0)	(128.0)	80.8	(94.5)
Pre-tax other adjustments		(1.8)		14.8
Certain earnings adjustments related to equity method investments and noncontrolling interest	$ (15.3)	$ (5.2)	$ (28.6)	$ (18.1)

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

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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 pre-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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The sum of income (expense), including income (expense) attributable to the noncontrolling interest, before income taxes for continuing operations.

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

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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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Aggregate revenue recognized during the period on a tax indemnification.

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Pre-tax 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 income before income taxes for pre-tax net realized capital gains (losses), as adjusted, pre-tax other adjustments management believes are not indicative of overall operating trends and certain adjustments related to equity method investments.

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Aggregate earnings recognized during the period for certain adjustments related to equity method investments and noncontrolling interest.

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Aggregate revenue recognized during the period for certain adjustments related to equity method investments.

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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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The pre-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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Pre-tax net realized capital gains (losses) are net of related changes in the amortization pattern of deferred acquisition costs and related actuarial balances, recognition of deferred front-end fee revenues for sales charges on retirement and life insurance products and services, amortization of hedge accounting book value adjustments for certain discontinued hedges, net realized capital gains and losses distributed, noncontrolling interest capital gains and losses, certain adjustments related to seed money and certain market value adjustments to fee revenues. Pre-tax 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 and realized capital gains (losses) associated with our exited group medical insurance business.

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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

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