Segment Information - Income Tax Expense by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Information: Income tax expense (benefit)
Income taxes (benefits)	$ 177.6	$ 318.5	$ 187.9
Operating Segments
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	341.9	372.2	341.1
Operating Segments | Retirement and Income Solutions
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	76.1	128.5	139.2
Operating Segments | Principal Global Investors
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	148.4	132.9	109.5
Operating Segments | Principal International
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	51.5	84.7	79.2
Operating Segments | U.S. Insurance Solutions
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	143.1	112.8	94.0
Operating Segments | Corporate
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	(77.2)	(86.7)	(80.8)
Reconciling Items
Segment Information: Income tax expense (benefit)
Tax expense/benefit related to net realized capital gains (losses), as adjusted	(45.6)	(43.6)	(101.4)
Tax expense (benefit) related to other after-tax adjustments	(63.2)	44.3	(0.6)
Tax expense (benefit) associated with certain adjustments related to equity method investments and noncontrolling interest	$ (55.5)	$ (54.4)	$ (51.2)