Investments - Real Estate (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Real estate
Depreciation expense on invested real estate	$ 55.9	$ 52.0	$ 49.3
Accumulated depreciation on invested real estate	$ 451.6	$ 450.4

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