Segment Information - Reconciliation of Segment Assets to Consolidated (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Segment Information: Assets
Total assets	$ 296,627.7	$ 276,087.8
Retirement and Income Solutions
Segment Information: Assets
Total assets	207,288.4	192,698.1
Principal Global Investors
Segment Information: Assets
Total assets	2,294.3	2,363.3
Principal International
Segment Information: Assets
Total assets	51,707.6	48,857.6
U.S. Insurance Solutions
Segment Information: Assets
Total assets	31,438.9	28,669.6
Corporate
Segment Information: Assets
Total assets	$ 3,898.5	$ 3,499.2