Revenues from Contracts with Customers - Disaggregation by Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues from Contracts with Customers - Disaggregation of Revenues
Total fees and other revenues per consolidated statements of operations	$ 1,251.8	$ 1,143.0	$ 3,711.0	$ 3,332.6
Operating Segments
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	796.5	681.9	2,329.8	2,035.6
Operating Segments | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	146.6	135.5	441.0	447.2
Operating Segments | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	453.5	377.0	1,303.6	1,108.1
Fees and other revenues not within the scope of revenue recognition guidance	11.0	4.8	27.1	14.2
Total fees and other revenues per consolidated statements of operations	464.5	381.8	1,330.7	1,122.3
Operating Segments | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	125.1	111.1	373.5	322.2
Fees and other revenues not within the scope of revenue recognition guidance	1.0	1.4	3.1	4.2
Total fees and other revenues per consolidated statements of operations	126.1	112.5	376.6	326.4
Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	19.7	15.9	55.8	46.9
Operating Segments | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	51.6	42.4	155.9	111.2
Fees and other revenues not within the scope of revenue recognition guidance	(91.8)	(75.8)	(266.3)	(221.1)
Total fees and other revenues per consolidated statements of operations	(40.2)	(33.4)	(110.4)	(109.9)
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	144.2	133.4	433.8	441.0
Fees and other revenues not within the scope of revenue recognition guidance	324.1	293.5	964.3	829.1
Total fees and other revenues per consolidated statements of operations	468.3	426.9	1,398.1	1,270.1
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	2.4	2.1	7.2	6.2
Fees and other revenues not within the scope of revenue recognition guidance	2.1	1.9	6.8	7.8
Total fees and other revenues per consolidated statements of operations	4.5	4.0	14.0	14.0
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	3.9	11.4	11.2
Fees and other revenues not within the scope of revenue recognition guidance	5.4	4.9	14.9	14.4
Total fees and other revenues per consolidated statements of operations	9.3	8.8	26.3	25.6
Operating Segments | Individual Life insurance | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	15.8	12.0	44.5	35.8
Fees and other revenues not within the scope of revenue recognition guidance	204.1	232.6	635.8	634.4
Total fees and other revenues per consolidated statements of operations	219.9	244.6	680.3	670.2
Operating Segments | U.S. Insurance Solutions Eliminations | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers			(0.1)	(0.1)
Reconciling Items
Revenues from Contracts with Customers - Disaggregation of Revenues
Fees and other revenues not within the scope of revenue recognition guidance	455.9	463.3	1,385.7	1,283.0
Pre-tax other adjustments	$ (0.6)	$ (2.2)	$ (4.5)	$ 14.0