Segment Information - Depreciation and Amortization Expense by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	$ 166.8	$ 151.0	$ 139.8
Retirement and Income Solutions
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	48.2	32.5	30.6
Principal Global Investors
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	18.9	21.9	19.8
Principal International
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	58.6	59.6	53.1
U.S. Insurance Solutions
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	25.0	26.2	28.1
Corporate
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	$ 16.1	$ 10.8	$ 8.2