Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Company contributions		$ 27.7	$ 14.9	$ 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	$ 0.5	$ 0.9		$ 0.0