Segment Information - Reconciliation of Segment Assets to Consolidated (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Segment Information: Assets
Total assets	$ 223,074.2	$ 218,660.3
Retirement and Income Solutions
Segment Information: Assets
Total assets	148,097.3	139,678.5
Principal Global Investors
Segment Information: Assets
Total assets	1,853.4	1,880.4
Principal International
Segment Information: Assets
Total assets	45,249.2	50,588.6
U.S. Insurance Solutions
Segment Information: Assets
Total assets	23,101.3	22,156.9
Corporate
Segment Information: Assets
Total assets	$ 4,773.0	$ 4,355.9