Postretirement Benefits - Defined Contribution Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract]
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	4.50%
Employer contributions	$ 3.6	$ 3.6	$ 3.0