Segment Information (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Segment Information: Assets
Total assets	$ 216,422.8	$ 208,191.4
Retirement and Investor Services
Segment Information: Assets
Total assets	133,306.2	128,736.7
Principal Global Investors
Segment Information: Assets
Total assets	1,200.1	1,312.1
Principal International
Segment Information: Assets
Total assets	56,974.6	54,243.6
U.S. Insurance Solutions
Segment Information: Assets
Total assets	21,215.0	20,033.6
Corporate
Segment Information: Assets
Total assets	$ 3,726.9	$ 3,865.4