Income Taxes - Comprehensive Income Tax Benefit and Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Income Tax Disclosure [Abstract]
Income Tax Expense (Benefit)	$ 76.0	$ (74.8)	$ (84.4)
Unrealized (Depreciation) Appreciation on Securities	42.2	(50.3)	325.5
Tax Effects from Postretirement Benefit Plans	0.3	(12.4)	(4.0)
Tax Effects on changes in Discount Rate for Life Reserves	(58.3)	21.2	(289.9)
Tax Effects from Cash Flow Hedge	0.8	0.0	(1.2)
Comprehensive Income (Loss), Tax	$ 91.0	$ (33.3)	$ (114.8)