Segment Information - Depreciation and Amortization Expense by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	$ 139.8	$ 133.5	$ 143.7
Retirement and Income Solutions
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	30.6	30.4	28.3
Principal Global Investors
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	19.8	18.8	16.5
Principal International
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	53.1	50.1	69.3
U.S. Insurance Solutions
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	28.1	26.8	23.1
Corporate
Segment Information: Depreciation and amortization expense
Depreciation and amortization expense	$ 8.2	$ 7.4	$ 6.5