Segment Information - Income Tax Expense by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	$ 320.2	$ 357.0	$ 395.1
Tax expense (benefit) related to net realized capital gains (losses), as adjusted	1.3	(71.4)	209.1
Tax benefit related to other after-tax adjustments			(594.5)
Tax expense (benefit) associated with certain adjustments related to equity method investments and noncontrolling interest	(72.3)	(54.9)	(82.0)
Income taxes (benefits)	249.2	230.7	(72.3)
Reconciling Items
Segment Information: Income tax expense (benefit)
Tax benefit associated with the U.S. tax reform			568.3
Principal Life Insurance Company and other subsidiaries combined | Legal Entities
Segment Information: Income tax expense (benefit)
Income taxes (benefits)	305.2	293.0	(280.9)
Retirement and Income Solutions
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	71.8	85.6	123.0
Principal Global Investors
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	121.8	149.1	186.0
Principal International
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	106.8	73.3	87.5
U.S. Insurance Solutions
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	101.8	85.7	134.0
Corporate
Segment Information: Income tax expense (benefit)
Total segment income tax expense (benefit) from operating earnings (losses)	$ (82.0)	$ (36.7)	$ (135.4)