Deferred Acquisition Costs and Other Actuarial Balances - Disaggregated DAC amounts (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025	Dec. 31, 2024
Deferred Acquisition Costs
Deferred acquisition costs	$ 4,112.2	$ 4,071.6
Short-duration contracts
Deferred Acquisition Costs
Deferred acquisition costs	36.6	30.2
Other balances
Deferred Acquisition Costs
Deferred acquisition costs		1.3
Retirement and Income Solutions
Deferred Acquisition Costs
Deferred acquisition costs	1,043.7	1,016.5
Retirement and Income Solutions \| Workplace savings and retirement solutions
Deferred Acquisition Costs
Deferred acquisition costs	534.0	528.9	$ 515.5
Retirement and Income Solutions \| Individual variable annuities
Deferred Acquisition Costs
Deferred acquisition costs	415.8	385.3	323.4
Retirement and Income Solutions \| Pension risk transfer
Deferred Acquisition Costs
Deferred acquisition costs	26.6	24.8	21.1
Retirement and Income Solutions \| Individual fixed deferred annuities
Deferred Acquisition Costs
Deferred acquisition costs	54.8	66.9	84.2
Retirement and Income Solutions \| Investment only
Deferred Acquisition Costs
Deferred acquisition costs	12.5	10.6	13.0
Benefits and Protection
Deferred Acquisition Costs
Deferred acquisition costs	3,031.9	3,023.6
Benefits and Protection \| Specialty Benefits \| Individual disability
Deferred Acquisition Costs
Deferred acquisition costs excluding short-duration contracts and other balances	731.3	714.8	696.9
Benefits and Protection \| Life Insurance \| Universal life
Deferred Acquisition Costs
Deferred acquisition costs excluding short-duration contracts and other balances	1,506.3	1,514.3	1,527.7
Benefits and Protection \| Life Insurance \| Term life
Deferred Acquisition Costs
Deferred acquisition costs excluding short-duration contracts and other balances	728.0	723.5	710.8
Benefits and Protection \| Life Insurance \| Participating life
Deferred Acquisition Costs
Deferred acquisition costs excluding short-duration contracts and other balances	$ 66.3	$ 71.0	$ 77.8