Segment Information - Income Tax Expense by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Information: Income tax expense (benefit)
Income taxes (benefits)	$ (72.3)	$ 229.9	$ 177.6
Operating Segments
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	395.1	337.6	341.9
Operating Segments | Retirement and Income Solutions
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	109.5	98.6	76.1
Operating Segments | Principal Global Investors
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	181.9	166.8	148.4
Operating Segments | Principal International
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	81.8	62.7	51.5
Operating Segments | U.S. Insurance Solutions
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	124.2	118.8	143.1
Operating Segments | Corporate
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	(102.3)	(109.3)	(77.2)
Reconciling Items
Segment Information: Income tax expense (benefit)
Tax expense (benefit) related to net realized capital gains (losses), as adjusted	209.1	(6.6)	(45.6)
Tax expense (benefit) related to other after-tax adjustments	(594.5)	(34.4)	(63.2)
Tax expense (benefit) associated with certain adjustments related to equity method investments and noncontrolling interest	(82.0)	$ (66.7)	$ (55.5)
Tax benefit associated with the U.S. tax reform	$ 568.3