Income Taxes - Comprehensive Income Tax Benefit and Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating Loss Carryforwards [Line Items]
Income Tax Benefit (Expense)	$ 124.8	$ (100.2)	$ (130.5)
Unrealized Depreciation (Appreciation) on Securities	(60.1)	(78.3)	(85.2)
Tax Effects from Postretirement Benefit Plans	(2.4)	(15.3)	1.7
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax	0.1	0.0	(0.1)
Comprehensive Income Tax (Expense) Benefit	62.4	(193.8)	(214.1)
Operations
Operating Loss Carryforwards [Line Items]
Income Tax Benefit (Expense)	$ 124.8	$ (100.2)	$ (130.5)