Revenues from Contracts with Customers - Disaggregation by Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenues from Contracts with Customers - Disaggregation of Revenues
Total fees and other revenues per consolidated statements of operations	$ 1,006.6	$ 1,012.6	$ 1,980.1	$ 2,023.9
Operating Segments
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	587.2	614.3	1,148.4	1,217.1
Operating Segments | Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	66.2	69.6	127.5	134.0
Operating Segments | Principal Global Investors
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	346.2	371.1	678.5	731.4
Fees and other revenues not within the scope of revenue recognition guidance	10.4	7.3	14.7	14.0
Total fees and other revenues per consolidated statements of operations	356.6	378.4	693.2	745.4
Operating Segments | Principal International
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	116.2	123.1	231.1	244.2
Fees and other revenues not within the scope of revenue recognition guidance	1.7	1.8	3.4	3.8
Total fees and other revenues per consolidated statements of operations	117.9	124.9	234.5	248.0
Operating Segments | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	16.1	13.5	31.8	26.6
Operating Segments | Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	42.5	37.0	79.5	80.9
Fees and other revenues not within the scope of revenue recognition guidance	(77.9)	(78.7)	(148.5)	(157.3)
Total fees and other revenues per consolidated statements of operations	(35.4)	(41.7)	(69.0)	(76.4)
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	63.9	66.9	122.3	128.6
Fees and other revenues not within the scope of revenue recognition guidance	281.6	282.8	553.4	569.2
Total fees and other revenues per consolidated statements of operations	345.5	349.7	675.7	697.8
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	2.3	2.7	5.2	5.4
Fees and other revenues not within the scope of revenue recognition guidance	4.2	2.9	9.1	5.6
Total fees and other revenues per consolidated statements of operations	6.5	5.6	14.3	11.0
Operating Segments | Specialty Benefits insurance | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	3.6	3.8	7.3	7.5
Fees and other revenues not within the scope of revenue recognition guidance	4.9	5.2	9.8	10.4
Total fees and other revenues per consolidated statements of operations	8.5	9.0	17.1	17.9
Operating Segments | Individual Life insurance | U.S. Insurance Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	12.6	9.8	24.6	19.2
Fees and other revenues not within the scope of revenue recognition guidance	190.3	177.7	382.5	363.4
Total fees and other revenues per consolidated statements of operations	202.9	187.5	407.1	382.6
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)	(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	415.2	399.0	824.4	809.1
Pre-tax other adjustments	$ 4.2	$ (0.7)	$ 7.3	$ (2.3)