Segment Information - Reconciliation of Segment Assets to Consolidated (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Segment Information: Assets
Total assets	$ 217,570.0	$ 218,660.3
Retirement and Income Solutions
Segment Information: Assets
Total assets	144,101.7	139,678.5
Principal Global Investors
Segment Information: Assets
Total assets	1,792.1	1,880.4
Principal International
Segment Information: Assets
Total assets	44,523.5	50,588.6
U.S. Insurance Solutions
Segment Information: Assets
Total assets	22,596.4	22,156.9
Corporate
Segment Information: Assets
Total assets	$ 4,556.3	$ 4,355.9