Segment Information (Details 4) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Information: Income tax expense (benefit)
Tax expense/benefit related to net realized capital gains (losses), as adjusted	$ (74.3)	$ (130.8)	$ (129.5)
Tax expense (benefit) related to other after-tax adjustments	63.3	21.0	37.5
Income taxes (benefits)	236.4	124.1	100.1
Aggregate Segments
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)	247.4	233.9	192.1
Retirement and Investor Services
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)	173.7	170.5	144.2
Principal Global Investors
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)	41.0	33.7	21.0
Principal International
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)	4.0	(0.4)	0.5
U.S. Insurance Solutions
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)	101.6	91.5	97.8
Corporate
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)	$ (72.9)	$ (61.4)	$ (71.4)