Deferred Acquisition Costs and Other Actuarial Balances - Benefits and Protection - Balances and changes in DAC (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2025	Dec. 31, 2024
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	$ 4,006.9
Ending balance, Deferred acquisition costs	4,048.0	$ 4,006.9
Benefits and Protection
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	3,013.2
Ending balance, Deferred acquisition costs	3,019.3	3,013.2
Individual disability \| Benefits and Protection \| Specialty Benefits
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	696.9	667.7
Costs deferred	35.1	79.0
Amortized to expense	(26.8)	(49.8)
Ending balance, Deferred acquisition costs	705.2	696.9
Universal life \| Benefits and Protection \| Life Insurance
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	1,527.7	1,545.3
Costs deferred	43.4	76.7
Amortized to expense	(47.5)	(94.3)
Ending balance, Deferred acquisition costs	1,523.6	1,527.7
Term life \| Benefits and Protection \| Life Insurance
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	710.8	695.1
Costs deferred	36.7	78.3
Amortized to expense	(31.4)	(62.6)
Ending balance, Deferred acquisition costs	716.1	710.8
Participating life \| Benefits and Protection \| Life Insurance
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	77.8	84.7
Costs deferred	0.7	1.7
Amortized to expense	(4.1)	(8.6)
Ending balance, Deferred acquisition costs	$ 74.4	$ 77.8