Employee and Agent Benefits - Other Postretirement Plan Changes and Plan Gains/Losses (Details) - Other postretirement benefits $ in Millions	Jan. 01, 2021 USD ($)
Defined Benefit Plan Disclosures
Amount of assets in excess of expected liability for retirees, re-designated for non-retiree benefits	$ 655.5
Percentage of assets in excess of expected liabilities to fund other benefits covered under the plans	125.00%

Represents the percentage that assets must exceed the expected liability for retirees before excess assets may be re-designated to fund other benefits covered by the plan pursuant to plan provisions.

No definition available.

Name:	pfg_DefinedBenefitPlanPostretirementMedicalPlanPercentageOfAssetsInExcessOfExpectedLiabilityForRetireesReDesignatedToNonRetirees
Namespace Prefix:	pfg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant