Revenues from Contracts with Customers - Disaggregation by Segment (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Revenues from Contracts with Customers - Disaggregation of Revenues
Total fees and other revenues per consolidated statements of operations	$ 1,144.1	$ 1,128.7	$ 3,407.4	$ 3,289.0
Operating Segments
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	752.4	779.8	2,309.3	2,275.6
Operating Segments \| Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	147.1	155.4	479.6	449.3
Adjustments for fees and other revenues not within the scope of revenue recognition guidance	324.9	303.9	913.8	885.4
Total fees and other revenues per consolidated statements of operations	472.0	459.3	1,393.4	1,334.7
Operating Segments \| Principal Asset Management
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	521.2	532.5	1,575.1	1,566.8
Operating Segments \| Benefits and Protection
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	15.4	27.0	46.2	78.5
Operating Segments \| Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	68.7	64.9	208.4	181.0
Adjustments for fees and other revenues not within the scope of revenue recognition guidance	(66.7)	(81.4)	(205.3)	(238.4)
Total fees and other revenues per consolidated statements of operations	2.0	(16.5)	3.1	(57.4)
Operating Segments \| Investment Management \| Principal Asset Management
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	436.3	448.6	1,309.5	1,308.4
Adjustments for fees and other revenues not within the scope of revenue recognition guidance	14.3	8.9	32.4	21.5
Total fees and other revenues per consolidated statements of operations	450.6	457.5	1,341.9	1,329.9
Operating Segments \| International Pension \| Principal Asset Management
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	90.6	88.7	282.9	272.5
Adjustments for fees and other revenues not within the scope of revenue recognition guidance	1.0	5.7	3.0	8.4
Total fees and other revenues per consolidated statements of operations	91.6	94.4	285.9	280.9
Operating Segments \| Specialty Benefits \| Benefits and Protection
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	3.3	4.0	10.6	11.7
Adjustments for fees and other revenues not within the scope of revenue recognition guidance	4.7	4.4	13.2	13.1
Total fees and other revenues per consolidated statements of operations	8.0	8.4	23.8	24.8
Operating Segments \| Life Insurance \| Benefits and Protection
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	12.2	23.1	35.7	66.9
Adjustments for fees and other revenues not within the scope of revenue recognition guidance	95.8	89.7	287.6	271.1
Total fees and other revenues per consolidated statements of operations	108.0	112.8	323.3	338.0
Operating Segments \| Benefits And Protection Eliminations \| Benefits and Protection
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	(0.1)	(0.1)	(0.1)	(0.1)
Operating Segments \| Principal Asset Management Eliminations \| Principal Asset Management
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	(5.7)	(4.8)	(17.3)	(14.1)
Reconciling items
Revenues from Contracts with Customers - Disaggregation of Revenues
Adjustments for fees and other revenues not within the scope of revenue recognition guidance	374.1	331.2	1,044.7	961.1
Pre-tax other adjustments	$ 17.6	$ 17.7	$ 53.4	$ 52.3