Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Sep. 30, 2023
Defined Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Company contributions	$ 12.0	$ 8.0	$ 32.6
Defined Benefit Pension Plan | Minimum
Defined Benefit Plan Disclosure [Line Items]
Total expected contributions for current fiscal year	35.0
Defined Benefit Pension Plan | Maximum
Defined Benefit Plan Disclosure [Line Items]
Total expected contributions for current fiscal year	45.0
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Amortization of actuarial gain	$ 0.1	$ 0.6