Segment Information (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Segment Information: Assets
Total assets	$ 201,687.4	$ 161,830.2
Retirement and Investor Services
Segment Information: Assets
Total assets	124,202.2	117,399.5
Principal Global Investors
Segment Information: Assets
Total assets	1,273.8	1,282.2
Principal International
Segment Information: Assets
Total assets	53,158.2	19,170.9
U.S. Insurance Solutions
Segment Information: Assets
Total assets	19,754.7	19,017.2
Corporate
Segment Information: Assets
Total assets	$ 3,298.5	$ 4,960.4