Segment Information (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Segment Information: Assets
Total assets	$ 222,270.7	$ 219,087.0
Retirement and Investor Services
Segment Information: Assets
Total assets	142,053.8	138,549.4
Principal Global Investors
Segment Information: Assets
Total assets	1,112.0	1,175.1
Principal International
Segment Information: Assets
Total assets	52,865.8	53,531.8
U.S. Insurance Solutions
Segment Information: Assets
Total assets	22,071.2	21,554.5
Corporate
Segment Information: Assets
Total assets	$ 4,167.9	$ 4,276.2