Postretirement Benefits - Determination of Pension Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Investment gains or losses recognition period	5 years
Cumulative unrecognized net actuarial gains (loss)	$ 0.9
Cumulative unrecognized investment gains	$ 0.5	$ 0.5
Maximum
Defined Benefit Plan Disclosure [Line Items]
Cumulative unrecognized net actuarial gains (loss)		$ (0.1)