Revenues from Contracts with Customers - Disaggregation by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues from Contracts with Customers - Disaggregation of Revenues
Total fees and other revenues per consolidated statements of operations	$ 4,273.8	$ 3,892.3	$ 3,627.4
Operating Segments
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	2,658.9	2,299.2	2,191.0
Operating Segments | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	262.9	268.2	267.9
Operating Segments | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	1,700.5	1,406.1	1,348.9
Fees and other revenues not within the scope of revenue recognition guidance	30.4	28.1	24.2
Total fees and other revenues per consolidated statements of operations	1,730.9	1,434.2	1,373.1
Operating Segments | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	477.1	426.4	398.7
Fees and other revenues not within the scope of revenue recognition guidance	7.3	7.1	8.7
Total fees and other revenues per consolidated statements of operations	484.4	433.5	407.4
Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	57.0	50.1	49.8
Operating Segments | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	161.4	148.4	125.7
Fees and other revenues not within the scope of revenue recognition guidance	(316.3)	(292.8)	(275.8)
Total fees and other revenues per consolidated statements of operations	(154.9)	(144.4)	(150.1)
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	252.4	258.0	259.1
Fees and other revenues not within the scope of revenue recognition guidance	1,132.9	1,111.2	1,021.5
Total fees and other revenues per consolidated statements of operations	1,385.3	1,369.2	1,280.6
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	10.5	10.2	8.8
Fees and other revenues not within the scope of revenue recognition guidance	12.5	15.0	15.3
Total fees and other revenues per consolidated statements of operations	23.0	25.2	24.1
Operating Segments | Specialty Benefits insurance | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	14.8	13.9	13.4
Fees and other revenues not within the scope of revenue recognition guidance	20.6	21.3	22.6
Total fees and other revenues per consolidated statements of operations	35.4	35.2	36.0
Operating Segments | Individual Life insurance | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	42.4	36.4	36.6
Fees and other revenues not within the scope of revenue recognition guidance	727.0	703.5	622.2
Total fees and other revenues per consolidated statements of operations	769.4	739.9	658.8
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	1,614.4	1,593.4	1,438.7
Pre-tax other adjustments	$ 0.5	$ (0.3)	$ (2.3)