Income Taxes - Comprehensive Income Tax Benefit and Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Operating Loss Carryforwards [Line Items]
Income Tax Benefit (Expense)	$ 88.3	$ 124.8	$ (100.2)
Unrealized Depreciation (Appreciation) on Securities	325.5	(60.1)	(78.3)
Tax Effects from Postretirement Benefit Plans	(4.0)	(2.4)	(15.3)
Tax Effects from Cash Flow Hedge	(1.2)	0.1	0.0
Comprehensive Income Tax Benefit (Expense)	408.6	62.4	(193.8)
Operations
Operating Loss Carryforwards [Line Items]
Income Tax Benefit (Expense)	$ 88.3	$ 124.8	$ (100.2)