Deferred Acquisition Costs and Other Actuarial Balances - Benefits and Protection - Balances and changes in DAC (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	$ 3,950.5
Ending balance, Deferred acquisition costs	3,965.5	$ 3,950.5
Individual disability | Benefits and Protection | Specialty Benefits
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	667.7	626.1
Costs deferred	20.2	86.7
Amortized to expense	(11.7)	(45.1)
Ending balance, Deferred acquisition costs	676.2	667.7
Universal life | Benefits and Protection | Life Insurance
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	1,545.3	1,569.7
Costs deferred	18.3	70.7
Amortized to expense	(23.4)	(95.1)
Ending balance, Deferred acquisition costs	1,540.2	1,545.3
Term life | Benefits and Protection | Life Insurance
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	695.1	685.7
Costs deferred	18.4	71.3
Amortized to expense	(15.9)	(61.9)
Ending balance, Deferred acquisition costs	697.6	695.1
Participating life | Benefits and Protection | Life Insurance
Deferred Acquisition Costs
Beginning balance, Deferred acquisition costs	84.7	93.0
Costs deferred	0.7	1.4
Amortized to expense	(2.2)	(9.7)
Ending balance, Deferred acquisition costs	$ 83.2	$ 84.7