Segment Information - Reconciliation of Segment Assets to Consolidated (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Segment Information: Assets
Total assets	$ 240,153.9	$ 228,014.3
Retirement and Income Solutions
Segment Information: Assets
Total assets	162,151.2	152,721.7
Principal Global Investors
Segment Information: Assets
Total assets	2,392.0	1,952.1
Principal International
Segment Information: Assets
Total assets	47,476.7	45,118.3
U.S. Insurance Solutions
Segment Information: Assets
Total assets	24,112.6	23,144.2
Corporate
Segment Information: Assets
Total assets	$ 4,021.4	$ 5,078.0