Revenues from Contracts with Customers - Disaggregation by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues from Contracts with Customers - Disaggregation of Revenues
Total fees and other revenues per consolidated statements of operations	$ 4,409.9	$ 4,273.8	$ 3,892.3
Operating Segments
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	2,610.0	2,658.9	2,299.2
Operating Segments | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	464.8	262.9	268.2
Operating Segments | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	1,456.7	1,700.5	1,406.1
Fees and other revenues not within the scope of revenue recognition guidance	38.5	30.4	28.1
Total fees and other revenues per consolidated statements of operations	1,495.2	1,730.9	1,434.2
Operating Segments | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	462.2	477.1	426.4
Fees and other revenues not within the scope of revenue recognition guidance	6.3	7.3	7.1
Total fees and other revenues per consolidated statements of operations	468.5	484.4	433.5
Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	65.9	57.0	50.1
Operating Segments | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	160.4	161.4	148.4
Fees and other revenues not within the scope of revenue recognition guidance	(301.8)	(316.3)	(292.8)
Total fees and other revenues per consolidated statements of operations	(141.4)	(154.9)	(144.4)
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	455.5	252.4	258.0
Fees and other revenues not within the scope of revenue recognition guidance	1,139.0	1,132.9	1,111.2
Total fees and other revenues per consolidated statements of operations	1,594.5	1,385.3	1,369.2
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	9.3	10.5	10.2
Fees and other revenues not within the scope of revenue recognition guidance	13.9	12.5	15.0
Total fees and other revenues per consolidated statements of operations	23.2	23.0	25.2
Operating Segments | Specialty Benefits insurance | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	14.9	14.8	13.9
Fees and other revenues not within the scope of revenue recognition guidance	19.6	20.6	21.3
Total fees and other revenues per consolidated statements of operations	34.5	35.4	35.2
Operating Segments | Individual Life insurance | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	51.2	42.4	36.4
Fees and other revenues not within the scope of revenue recognition guidance	875.9	727.0	703.5
Total fees and other revenues per consolidated statements of operations	927.1	769.4	739.9
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	(0.2)	(0.2)	(0.2)
Reconciling Items
Revenues from Contracts with Customers - Disaggregation of Revenues
Fees and other revenues not within the scope of revenue recognition guidance	1,791.4	1,614.4	1,593.4
Pre-tax other adjustments	$ 8.5	$ 0.5	$ (0.3)