Segment Information - Reconciliation of Segment Assets to Consolidated (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Segment Information: Assets
Total assets	$ 271,309.4	$ 276,087.8
Retirement and Income Solutions
Segment Information: Assets
Total assets	191,053.8	192,698.1
Principal Global Investors
Segment Information: Assets
Total assets	2,197.2	2,363.3
Principal International
Segment Information: Assets
Total assets	45,381.9	48,857.6
U.S. Insurance Solutions
Segment Information: Assets
Total assets	29,745.6	28,669.6
Corporate
Segment Information: Assets
Total assets	$ 2,930.9	$ 3,499.2