Postretirement Benefits - Weighted-Average Assumption Used to Determine Net Cost (Details)	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract]
Discount rate (as a percent)	0.37%	0.55%	1.04%
Expected return on plan assets (as a percent)	1.02%	1.01%	1.06%
Expected rate of compensation increases (as a percent)	1.12%	1.12%	1.19%