Segment Information - Reconciliation of Segment Assets to Consolidated (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Segment Information: Assets
Total assets	$ 298,908.4	$ 296,627.7
Retirement and Income Solutions
Segment Information: Assets
Total assets	216,039.3	207,288.4
Principal Global Investors
Segment Information: Assets
Total assets	2,358.4	2,294.3
Principal International
Segment Information: Assets
Total assets	43,524.9	51,707.6
U.S. Insurance Solutions
Segment Information: Assets
Total assets	32,702.7	31,438.9
Corporate
Segment Information: Assets
Total assets	$ 4,283.1	$ 3,898.5