Segment Information (Details 4) (USD $) In Millions, unless otherwise specified	3 Months Ended				12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)					$ 289.9	$ 185.8	$ 224.2
Tax expense/benefit related to net realized capital gains (losses), as adjusted					(101.4)	(8.6)	(70.6)
Tax expense (benefit) related to other after-tax adjustments					(0.6)	(42.6)	44.7
Income taxes (benefits)	34.2	(7.2)	50.9	56.7	187.9	134.6	198.3
Aggregate Segments
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)					289.9	185.8	224.2
Retirement and Investor Services
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)					195.6	150.0	163.9
Principal Global Investors
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)					55.5	43.5	41.0
Principal International
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)					28.0	0.6	(3.1)
U.S. Insurance Solutions
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)					94.0	61.3	95.3
Corporate
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)					$ (83.2)	$ (69.6)	$ (72.9)