Postretirement Benefits - Determination of Pension Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Investment gains or losses recognition period	5 years
Cumulative unrecognized investment losses		$ 0.9
Cumulative unrecognized net actuarial gains	$ 0.4	$ (0.3)
Maximum
Defined Benefit Plan Disclosure [Line Items]
Cumulative unrecognized investment losses	$ 0.1