Segment Information (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Segment Information: Assets
Total assets	$ 211,193.9	$ 208,191.4
Retirement and Investor Services
Segment Information: Assets
Total assets	131,393.7	128,736.7
Principal Global Investors
Segment Information: Assets
Total assets	1,165.3	1,312.1
Principal International
Segment Information: Assets
Total assets	54,191.4	54,243.6
U.S. Insurance Solutions
Segment Information: Assets
Total assets	20,602.8	20,033.6
Corporate
Segment Information: Assets
Total assets	$ 3,840.7	$ 3,865.4