Postretirement Benefits - Defined Contribution Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Compensation and Retirement Disclosure [Abstract]
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	4.50%
Employer contributions	$ 5.2	$ 4.6	$ 3.4