Segment Information - Reconciliation of Segment Assets to Consolidated (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Segment Information: Assets
Total assets	$ 276,087.8	$ 243,036.1
Retirement and Income Solutions
Segment Information: Assets
Total assets	192,698.1	163,833.6
Principal Global Investors
Segment Information: Assets
Total assets	2,363.3	2,301.0
Principal International
Segment Information: Assets
Total assets	48,857.6	46,701.2
U.S. Insurance Solutions
Segment Information: Assets
Total assets	28,669.6	25,388.2
Corporate
Segment Information: Assets
Total assets	$ 3,499.2	$ 4,812.1