Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Sep. 30, 2020
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Company contributions			$ 34.0	$ 24.6	$ 37.5
Defined Benefit Pension Plan | Minimum
Defined Benefit Plan Disclosure [Line Items]
Total expected contributions for current fiscal year	$ 45.0		45.0
Defined Benefit Pension Plan | Maximum
Defined Benefit Plan Disclosure [Line Items]
Total expected contributions for current fiscal year	55.0		55.0
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Amortization of actuarial gain	$ 0.4	$ 0.0	$ 1.3