Segment Information (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Segment Information: Assets
Total assets	$ 152,050.7	$ 147,361.7
Retirement and Investor Services
Segment Information: Assets
Total assets	111,536.2	108,998.0
Principal Global Investors
Segment Information: Assets
Total assets	1,218.4	1,833.3
Principal International
Segment Information: Assets
Total assets	17,229.1	15,612.1
U.S. Insurance Solutions
Segment Information: Assets
Total assets	18,082.5	17,389.1
Corporate
Segment Information: Assets
Total assets	$ 3,984.5	$ 3,529.2