Investments - Real Estate (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Real estate
Depreciation expense on invested real estate	$ 65.3	$ 60.0	$ 54.1
Accumulated depreciation on invested real estate	$ 591.2	$ 526.0

The amount as of the balance sheet date of accumulated depreciation pertaining to invested real estate.

No definition available.

Name:	pfg_RealEstateInvestmentAccumulatedDepreciation
Namespace Prefix:	pfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant