Segment Information - Reconciliation of Segment Assets to Consolidated (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Segment Information: Assets
Total assets	$ 278,484.8	$ 276,087.8
Retirement and Income Solutions
Segment Information: Assets
Total assets	196,631.2	192,698.1
Principal Global Investors
Segment Information: Assets
Total assets	2,228.3	2,363.3
Principal International
Segment Information: Assets
Total assets	45,833.6	48,857.6
U.S. Insurance Solutions
Segment Information: Assets
Total assets	30,375.9	28,669.6
Corporate
Segment Information: Assets
Total assets	$ 3,415.8	$ 3,499.2