Segment Information - Depreciation and Amortization Expense by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	$ 186.6	$ 166.8	$ 151.0
Retirement and Income Solutions
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	69.1	48.2	32.5
Principal Global Investors
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	21.6	18.9	21.9
Principal International
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	56.0	58.6	59.6
U.S. Insurance Solutions
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	25.1	25.0	26.2
Corporate
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	$ 14.8	$ 16.1	$ 10.8