Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Net Income (Loss)	$ (75.3)	$ 122.3	$ (14.7)	$ 312.4
Other Comprehensive Income (Loss) Before Income Taxes:
Decrease (Increase) in Net Unrecognized Postretirement Benefit Costs	(0.1)	0.9	0.3	2.3
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), before Reclassification and Tax	0.1	0.2	0.3	0.3
Total Other Comprehensive Income (Loss) Before Income Taxes	(80.3)	65.5	(255.3)	269.5
Other Comprehensive Income Tax Benefit (Expense)	16.7	(13.8)	54.2	(56.6)
Other Comprehensive Income (Loss), Net of Taxes	(63.6)	51.7	(201.1)	212.9
Total Comprehensive Income (Loss)	(138.9)	174.0	(215.8)	525.3
No Credit Losses Recognized in Condensed Consolidated Statements of Income
Other Comprehensive Income (Loss) Before Income Taxes:
Changes in Net Unrealized Holding Gains (Losses) on Investment Securities with:	(80.8)	63.1	(254.0)	269.3
Credit Losses Recognized in Condensed Consolidated Statements of Income
Other Comprehensive Income (Loss) Before Income Taxes:
Changes in Net Unrealized Holding Gains (Losses) on Investment Securities with:	$ 0.5	$ 1.3	$ (1.9)	$ (2.4)