Segment Information - Reconciliation of Segment Assets to Consolidated (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Segment Information: Assets
Total assets	$ 253,636.5	$ 253,941.2
Retirement and Income Solutions
Segment Information: Assets
Total assets	168,851.9	169,757.8
Principal Global Investors
Segment Information: Assets
Total assets	2,234.5	2,322.9
Principal International
Segment Information: Assets
Total assets	52,923.4	51,684.0
U.S. Insurance Solutions
Segment Information: Assets
Total assets	25,118.4	25,092.9
Corporate
Segment Information: Assets
Total assets	$ 4,508.3	$ 5,083.6