Segment Information (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Segment Information: Assets
Total assets	$ 159,193.3	$ 147,361.7
Retirement and Investor Services
Segment Information: Assets
Total assets	116,138.7	108,998.0
Principal Global Investors
Segment Information: Assets
Total assets	1,265.1	1,833.3
Principal International
Segment Information: Assets
Total assets	18,849.7	15,612.1
U.S. Insurance Solutions
Segment Information: Assets
Total assets	18,682.3	17,389.1
Corporate
Segment Information: Assets
Total assets	$ 4,257.5	$ 3,529.2