Segment Information - Reconciliation of Segment Assets to Consolidated (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Segment Information: Assets
Total assets	$ 235,496.0	$ 228,014.3
Retirement and Income Solutions
Segment Information: Assets
Total assets	158,505.5	152,721.7
Principal Global Investors
Segment Information: Assets
Total assets	1,866.8	1,952.1
Principal International
Segment Information: Assets
Total assets	47,440.1	45,118.3
U.S. Insurance Solutions
Segment Information: Assets
Total assets	23,553.3	23,144.2
Corporate
Segment Information: Assets
Total assets	$ 4,130.3	$ 5,078.0