Segment Information (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Segment Information: Assets
Total assets	$ 148,298.0	$ 145,631.1
Retirement and Investor Services
Segment Information: Assets
Total assets	109,481.8	110,043.0
Principal Global Investors
Segment Information: Assets
Total assets	1,833.3	1,308.1
Principal International
Segment Information: Assets
Total assets	15,662.8	12,774.5
U.S. Insurance Solutions
Segment Information: Assets
Total assets	17,790.9	16,558.2
Corporate
Segment Information: Assets
Total assets	$ 3,529.2	$ 4,947.3