Goodwill and Other Intangible Assets (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Finite Lived Intangible Asset Disclosures
Weighted average remaining expected life of finite lived intangible assets (in years)	13
Gross carrying amount	$ 410.6	$ 343.0
Accumulated amortization	187.2	177.2
Net carrying amount	223.4	165.8
Present value of future profits
Finite Lived Intangible Asset Disclosures
Gross carrying amount	191.7	148.7
Accumulated amortization	47.9	48.7
Net carrying amount	143.8	100.0
Present value of future profits | Principal International
Estimated amortization expense, net of interest accrued, related to present value of future profits for next five years
Year 1: Present value of future profits amortization	2.2
Year 2: Present value of future profits amortization	2.3
Year 3: Present value of future profits amortization	3.3
Year 4: Present value of future profits amortization	4.3
Year 5: Present value of future profits amortization	5.2
Changes in Present Value of Future Profits
Beginning balance, Present value of future profits	100.0	98.4	84.4
Acquisitions, Present value of future profits	67.4
Interest accrued, Present value of future profits	9.4	8.0	7.6
Amortization, Present value of future profits	(14.2)	(11.5)	(8.9)
Foreign currency translation, Present value of future profits	(18.8)	5.1	5.1
Other changes, Present value of future profits			10.2
Ending balance, Present value of future profits	143.8	100.0	98.4
Other finite lived intangible assets
Finite Lived Intangible Asset Disclosures
Gross carrying amount	218.9	194.3
Accumulated amortization	139.3	128.5
Net carrying amount	79.6	65.8
Finite lived intangible assets fully amortized during the period		1.7
Estimated amortization expense for the next five years
Year 1: Finite lived intangible assets amortization	13.3
Year 2: Finite lived intangible assets amortization	11.5
Year 3: Finite lived intangible assets amortization	10.5
Year 4: Finite lived intangible assets amortization	8.9
Year 5: Finite lived intangible assets amortization	8.8
Other Finite Lived Intangible Assets
Amortization expense for intangible assets with finite useful lives	11.3	18.9	35.2
Other finite lived intangible assets | WM Advisors, Inc.
Other Finite Lived Intangible Assets
Other finite lived intangible assets impairment			(6.5)
Other finite lived intangible assets | Group medical insurance
Finite Lived Intangible Asset Disclosures
Gross carrying amount		6.0
Accumulated amortization		4.4
Other Finite Lived Intangible Assets
Other finite lived intangible assets impairment		$ (1.6)