Segment Information (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Segment Information: Assets
Total assets	$ 200,762.9	$ 161,830.2
Retirement and Investor Services
Segment Information: Assets
Total assets	120,416.2	117,399.5
Principal Global Investors
Segment Information: Assets
Total assets	1,207.2	1,282.2
Principal International
Segment Information: Assets
Total assets	56,122.5	19,170.9
U.S. Insurance Solutions
Segment Information: Assets
Total assets	19,518.6	19,017.2
Corporate
Segment Information: Assets
Total assets	$ 3,498.4	$ 4,960.4