Employee and Agent Benefits - Reconciliation of Fair Value of Plan Assets using Level 3 Inputs (Details) - Other postretirement benefits - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Change in Level 3 fair value measurement rollforward, plan assets
Fair value of plan assets at beginning of year	$ 627.0
Fair value of plan assets at end of year	628.6	$ 627.0
Fair value hierarchy Level 3 \| Principal Life general account investment
Change in Level 3 fair value measurement rollforward, plan assets
Fair value of plan assets at beginning of year	33.5	36.3
Actual return gains (losses) on plan assets relating to assets still held at the reporting date	(1.7)	0.2
Actual return gains (losses) on plan assets relating to assets sold during the period	(33.6)
Net purchases, sales and settlements	$ 1.8	(3.0)
Fair value of plan assets at end of year		$ 33.5

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets sold.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(iv)(02)(A)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets still held.

+ References

+ Details

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- Definition

Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from purchases, sales and settlements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(iv)(02)(B)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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