Segment Information (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Segment Information: Assets
Total assets	$ 215,365.4	$ 219,087.0
Retirement and Investor Services
Segment Information: Assets
Total assets	137,404.4	138,549.4
Principal Global Investors
Segment Information: Assets
Total assets	1,149.9	1,175.1
Principal International
Segment Information: Assets
Total assets	50,458.0	53,531.8
U.S. Insurance Solutions
Segment Information: Assets
Total assets	22,180.7	21,554.5
Corporate
Segment Information: Assets
Total assets	$ 4,172.4	$ 4,276.2