Segment Information - Reconciliation of Segment Assets to Consolidated (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Segment Information: Assets
Total assets	$ 243,036.1	$ 253,941.2
Retirement and Income Solutions
Segment Information: Assets
Total assets	163,833.6	169,757.8
Principal Global Investors
Segment Information: Assets
Total assets	2,301.0	2,322.9
Principal International
Segment Information: Assets
Total assets	46,701.2	51,684.0
U.S. Insurance Solutions
Segment Information: Assets
Total assets	25,388.2	25,092.9
Corporate
Segment Information: Assets
Total assets	$ 4,812.1	$ 5,083.6