Segment Information - Reconciliation of Segment Assets to Consolidated (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Segment Information: Assets
Total assets	$ 246,903.9	$ 276,087.8
Retirement and Income Solutions
Segment Information: Assets
Total assets	174,996.9	192,698.1
Principal Global Investors
Segment Information: Assets
Total assets	2,173.2	2,363.3
Principal International
Segment Information: Assets
Total assets	39,445.7	48,857.6
U.S. Insurance Solutions
Segment Information: Assets
Total assets	27,928.3	28,669.6
Corporate
Segment Information: Assets
Total assets	$ 2,359.8	$ 3,499.2