Revenues from Contracts with Customers - Disaggregation by Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenues from Contracts with Customers - Disaggregation of Revenues
Total fees and other revenues per consolidated statements of operations	$ 1,143.0	$ 1,230.0	$ 3,332.6	$ 3,210.1
U.S. Insurance Solutions Eliminations
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers			(0.1)	(0.1)
Operating Segments
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	681.9	692.0	2,035.6	1,840.4
Operating Segments | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	135.5	158.3	447.2	285.8
Operating Segments | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	377.0	362.8	1,108.1	1,041.3
Fees and other revenues not within the scope of revenue recognition guidance	4.8	5.1	14.2	19.8
Total fees and other revenues per consolidated statements of operations	381.8	367.9	1,122.3	1,061.1
Operating Segments | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	111.1	114.8	322.2	345.9
Fees and other revenues not within the scope of revenue recognition guidance	1.4	1.6	4.2	5.0
Total fees and other revenues per consolidated statements of operations	112.5	116.4	326.4	350.9
Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	15.9	16.6	46.9	48.4
Operating Segments | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	42.4	39.5	111.2	119.0
Fees and other revenues not within the scope of revenue recognition guidance	(75.8)	(75.2)	(221.1)	(223.7)
Total fees and other revenues per consolidated statements of operations	(33.4)	(35.7)	(109.9)	(104.7)
Operating Segments | Retirement and Income Solutions - Fee | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	133.4	156.0	441.0	278.3
Fees and other revenues not within the scope of revenue recognition guidance	293.5	301.2	829.1	854.6
Total fees and other revenues per consolidated statements of operations	426.9	457.2	1,270.1	1,132.9
Operating Segments | Retirement and Income Solutions - Spread | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	2.1	2.3	6.2	7.5
Fees and other revenues not within the scope of revenue recognition guidance	1.9	2.6	7.8	11.7
Total fees and other revenues per consolidated statements of operations	4.0	4.9	14.0	19.2
Operating Segments | Specialty Benefits insurance | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	3.9	3.8	11.2	11.1
Fees and other revenues not within the scope of revenue recognition guidance	4.9	4.8	14.4	14.6
Total fees and other revenues per consolidated statements of operations	8.8	8.6	25.6	25.7
Operating Segments | Individual Life insurance | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	12.0	12.8	35.8	37.4
Fees and other revenues not within the scope of revenue recognition guidance	232.6	292.1	634.4	674.6
Total fees and other revenues per consolidated statements of operations	244.6	304.9	670.2	712.0
Reconciling Items
Revenues from Contracts with Customers - Disaggregation of Revenues
Fees and other revenues not within the scope of revenue recognition guidance	463.3	532.2	1,283.0	1,356.6
Pre-tax other adjustments	$ (2.2)	$ 5.8	$ 14.0	$ 13.1