Income Taxes - Comprehensive Income Tax Benefit and Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Operating Loss Carryforwards [Line Items]
Income Tax Benefit	$ 74.8	$ 84.4	[1]	$ 125.6	[1]
Unrealized (Appreciation) Depreciation on Securities	(50.3)	325.5		60.7
Tax Effects from Postretirement Benefit Plans	(12.4)	(4.0)		1.8
Tax Effects on changes in Discount Rate for Life Reserves	21.2	(289.9)		(47.9)
Tax Effects from Cash Flow Hedge	0.0	(1.2)		(0.1)
Comprehensive Income Tax Benefit	33.3	114.8		140.1
Operations
Operating Loss Carryforwards [Line Items]
Income Tax Benefit	$ 74.8	$ 84.4		$ 125.6

[1] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.