Revenues from Contracts with Customers - Disaggregation by Segment (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Revenues from Contracts with Customers - Disaggregation of Revenues
Total fees and other revenues per consolidated statements of operations	$ 1,146.0	$ 1,082.6	$ 2,263.3	$ 2,160.3
Operating Segments
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	787.0	750.0	1,556.9	1,495.8
Operating Segments \| Retirement and Income Solutions
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	181.9	148.2	332.5	293.9
Adjustments for fees and other revenues not within the scope of revenue recognition guidance	289.0	286.0	588.9	581.5
Total fees and other revenues per consolidated statements of operations	470.9	434.2	921.4	875.4
Operating Segments \| Principal Asset Management
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	526.8	519.6	1,053.9	1,034.3
Operating Segments \| Benefits and Protection
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	15.5	26.7	30.8	51.5
Operating Segments \| Corporate
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	62.8	55.5	139.7	116.1
Adjustments for fees and other revenues not within the scope of revenue recognition guidance	(61.3)	(78.1)	(138.6)	(157.0)
Total fees and other revenues per consolidated statements of operations	1.5	(22.6)	1.1	(40.9)
Operating Segments \| Investment Management \| Principal Asset Management
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	436.4	433.1	873.2	859.8
Adjustments for fees and other revenues not within the scope of revenue recognition guidance	13.0	9.8	18.1	12.6
Total fees and other revenues per consolidated statements of operations	449.4	442.9	891.3	872.4
Operating Segments \| International Pension \| Principal Asset Management
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	96.2	91.4	192.3	183.8
Adjustments for fees and other revenues not within the scope of revenue recognition guidance	1.0	1.5	2.0	2.7
Total fees and other revenues per consolidated statements of operations	97.2	92.9	194.3	186.5
Operating Segments \| Specialty Benefits \| Benefits and Protection
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	3.7	4.0	7.3	7.7
Adjustments for fees and other revenues not within the scope of revenue recognition guidance	4.2	4.1	8.5	8.7
Total fees and other revenues per consolidated statements of operations	7.9	8.1	15.8	16.4
Operating Segments \| Life Insurance \| Benefits and Protection
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	11.8	22.7	23.5	43.8
Adjustments for fees and other revenues not within the scope of revenue recognition guidance	95.2	91.8	191.8	181.4
Total fees and other revenues per consolidated statements of operations	107.0	114.5	215.3	225.2
Operating Segments \| Principal Asset Management Eliminations \| Principal Asset Management
Revenues from Contracts with Customers - Disaggregation of Revenues
Total segment revenue from contracts with customers	(5.8)	(4.9)	(11.6)	(9.3)
Reconciling items
Revenues from Contracts with Customers - Disaggregation of Revenues
Adjustments for fees and other revenues not within the scope of revenue recognition guidance	341.0	315.4	670.6	629.9
Pre-tax other adjustments	$ 18.0	$ 17.2	$ 35.8	$ 34.6