Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Net (Loss) Income	$ (301.2)	$ (120.5)	$ 409.9
Decrease (Increase) in Net Unrecognized Postretirement Benefit Costs	18.9	(8.8)	70.2
Gain on Cash Flow Hedges	5.9	0.5	0.4
Other Comprehensive (Loss) Income Before Income Taxes	(1,524.4)	(294.8)	437.9
Other Comprehensive Income Tax Benefit (Expense)	320.3	62.4	(93.5)
Other Comprehensive (Loss) Income, Net of Taxes	(1,204.1)	(232.4)	344.4
Total Comprehensive (Loss) Income	(1,505.3)	(352.9)	754.3
No Credit Losses
Changes in Net Unrealized Holding Gains (Losses) on Investment Securities	(1,551.1)	(284.5)	369.9
Credit Losses
Changes in Net Unrealized Holding Gains (Losses) on Investment Securities	$ 1.9	$ (2.0)	$ (2.6)