Goodwill and Other Intangible Assets (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended					12 Months Ended		12 Months Ended				12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Retirement and Investor Services	Dec. 31, 2010 Retirement and Investor Services	Dec. 31, 2009 Retirement and Investor Services	Dec. 31, 2011 Principal Global Investors	Dec. 31, 2009 Principal Global Investors	Dec. 31, 2011 Principal International	Dec. 31, 2010 Principal International	Dec. 31, 2011 U.S. Insurance Solutions	Dec. 31, 2009 U.S. Insurance Solutions	Dec. 31, 2011 Corporate	Dec. 31, 2010 Corporate	Dec. 31, 2010 Corporate Group medical insurance
Changes in Goodwill
Beginning balance, Goodwill	$ 345.4	$ 386.4	$ 72.6	$ 72.6	$ 72.6	$ 169.0	$ 169.0	$ 58.9	$ 56.3	$ 43.4	$ 43.4	$ 1.5	$ 45.1
Goodwill impairment		(43.6)											(43.6)	(43.6)
Goodwill from acquisitions	154.2					68.0		86.2
Foreign currency translation, Goodwill	(17.3)	4.2						(17.3)	4.2
Other changes in goodwill		(1.6)							(1.6)	1.5		(1.5)
Ending balance, Goodwill	$ 482.3	$ 345.4	$ 72.6	$ 72.6	$ 72.6	$ 237.0	$ 169.0	$ 127.8	$ 58.9	$ 44.9	$ 43.4		$ 1.5	$ 0