Investments - Net Unrealized Gains and Losses on Available-for-Sale Securities and Derivative Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Net Unrealized Gains and Losses on Available-for-Sale Securities and Derivative Instruments
Net unrealized losses on fixed maturities, available-for-sale	$ (5,585.1)	$ (5,143.0)
Net unrealized gains (losses) on derivative instruments	20.6	(1.6)
Adjustments for assumed changes in amortization patterns	(5.5)	(5.2)
Adjustments for assumed changes in policyholder liabilities	9.6	1.4
Net unrealized gains (losses) other investments and noncontrolling interest adjustments	21.0	43.1
Provision for deferred income tax benefits (taxes)	1,181.4	1,088.4
Net unrealized gains (losses) on available-for-sale securities and derivative instruments	$ (4,358.0)	$ (4,016.9)