Segment Information (Details 4) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Information: Income tax expense (benefit)
Tax expense/benefit related to net realized capital gains (losses), as adjusted	$ (43.6)	$ (101.4)	$ (8.6)
Income taxes (benefits)	318.5	187.9	134.6
Operating Segments
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)	317.8	289.9	185.8
Operating Segments | Retirement and Investor Services
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)	200.5	195.6	150.0
Operating Segments | Principal Global Investors
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)	63.8	55.5	43.5
Operating Segments | Principal International
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)	30.3	28.0	0.6
Operating Segments | U.S. Insurance Solutions
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)	112.8	94.0	61.3
Operating Segments | Corporate
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (loss)	(89.6)	(83.2)	(69.6)
Reconciling Items
Segment Information: Income tax expense (benefit)
Tax expense/benefit related to net realized capital gains (losses), as adjusted	(43.6)	(101.4)	(8.6)
Tax expense (benefit) related to other after-tax adjustments	$ 44.3	$ (0.6)	$ (42.6)