Segment Information - Reconciliation of Segment Assets to Consolidated (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Segment Information: Assets
Total assets	$ 247,933.6	$ 228,014.3
Retirement and Income Solutions
Segment Information: Assets
Total assets	166,508.0	152,721.7
Principal Global Investors
Segment Information: Assets
Total assets	2,417.1	1,952.1
Principal International
Segment Information: Assets
Total assets	49,704.6	45,118.3
U.S. Insurance Solutions
Segment Information: Assets
Total assets	24,496.8	23,144.2
Corporate
Segment Information: Assets
Total assets	$ 4,807.1	$ 5,078.0