Segment Information - Reconciliation of Segment Assets to Consolidated (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Segment Information: Assets
Total assets	$ 253,941.2	$ 228,014.3
Retirement and Income Solutions
Segment Information: Assets
Total assets	169,757.8	152,721.7
Principal Global Investors
Segment Information: Assets
Total assets	2,322.9	1,952.1
Principal International
Segment Information: Assets
Total assets	51,684.0	45,118.3
U.S. Insurance Solutions
Segment Information: Assets
Total assets	25,092.9	23,144.2
Corporate
Segment Information: Assets
Total assets	$ 5,083.6	$ 5,078.0