Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Income	$ 97.5	$ 122.3	$ 126.1	$ 64.0	$ 124.7	$ 129.0	$ 122.1	$ 155.3		$ 409.9	$ 531.1	$ 190.1
Foreign Currency Translation Adjustments												0.3
Decrease (Increase) in Net Unrecognized Postretirement Benefit Costs										(70.2)	7.8	6.9
Gain (Loss) on Cash Flow Hedges										0.4	0.4	1.2
Other Comprehensive Income (Loss) Before Income Taxes										437.9	397.9	(241.5)
Other Comprehensive Income Tax Benefit (Expense)										93.5	83.6	(50.7)
Other Comprehensive Income (Loss)										344.4	314.3	(190.8)
Total Comprehensive Income (Loss)										754.3	845.4	(0.7)
Income from Continuing Operations	97.5	122.3	126.1	64.0					$ 188.4	409.9	531.1	188.4
Income Tax Benefit (Expense)	$ (23.1)	$ (29.8)	$ (33.6)	$ (13.7)	$ (32.2)	$ (32.5)	$ (26.4)	$ (39.4)		(100.2)	(130.5)	(10.7)
No Credit Losses Recognized in Consolidated Statements of Income
Changes in Net Unrealized Holding Gains (Losses) on Investment Securities										369.9	$ 405.3	$ (236.1)
Credit Losses Recognized in Consolidated Statements of Income
Changes in Net Unrealized Holding Gains (Losses) on Investment Securities										$ (2.6)