Retirement Benefits Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jul. 31, 2021	Dec. 31, 2020
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accrued postretirement benefits	$ 269.4		$ 269.4			$ 326.7
Income tax provision	22.0	$ 0.8	31.5	$ 104.2
Settlement gain	(64.9)	0.0	(64.9)	0.0
Collective-Bargaining Arrangement, Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Income tax provision	15.5
Pension Plans Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year						$ 87.0
Defined Benefit Plan, Plan Assets, Contributions by Employer	50.0		67.5
Settlement gain	0.0	0.0	0.0	0.0
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Settlement gain	$ (64.9)	$ 0.0	$ (64.9)	$ 0.0
Other Postretirement Benefit Plan, Defined Benefit | Collective-Bargaining Arrangement, Other
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accrued postretirement benefits					$ 43.0
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax					$ 21.9