Postretirement Benefits - Determination of Pension Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Investment gains or losses recognition period	5 years
Cumulative unrecognized net actuarial gains		$ 0.4
Cumulative unrecognized investment gains (losses)	$ 0.5	$ (0.1)
Maximum
Defined Benefit Plan Disclosure [Line Items]
Cumulative unrecognized net actuarial gains	$ 0.1