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

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.

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

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- Definition

Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

+ References

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- Definition

Amount, measured using unobservable input, of increase (decrease) in plan asset of defined benefit plan from purchase, sale and settlement of trade associated with underlying investment.

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