Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Net Income	$ (62.6)	$ 126.1	$ 60.6	$ 190.1
Other Comprehensive Income (Loss) Before Income Taxes:
Decrease (Increase) in Net Unrecognized Postretirement Benefit Costs	0.1	0.7	0.4	1.4
Gain on Cash Flow Hedges	0.1	0.0	0.2	0.1
Total Other Comprehensive Income (Loss) Before Income Taxes	190.7	408.0	(175.0)	204.0
Other Comprehensive Income Tax Benefit (Expense)	(38.2)	(85.6)	37.5	(42.8)
Other Comprehensive Income (Loss), Net of Taxes	152.5	322.4	(137.5)	161.2
Total Comprehensive Income (Loss)	89.9	448.5	(76.9)	351.3
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:	190.8	408.1	(173.2)	206.2
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:	$ (0.3)	$ (0.8)	$ (2.4)	$ (3.7)