Segment Information (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Segment Information: Assets
Total assets	$ 153,738.2	$ 147,361.7
Retirement and Investor Services
Segment Information: Assets
Total assets	113,266.7	108,998.0
Principal Global Investors
Segment Information: Assets
Total assets	1,795.8	1,833.3
Principal International
Segment Information: Assets
Total assets	17,248.2	15,612.1
U.S. Insurance Solutions
Segment Information: Assets
Total assets	17,669.9	17,389.1
Corporate
Segment Information: Assets
Total assets	$ 3,757.6	$ 3,529.2