Revenue from Contracts with Clients - Narrative (Details)	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Revenues from Contracts With Clients [Abstract]
Total revenues (percentage)	1.40%	0.70%	1.40%	0.80%
Operating revenues (percentage)	32.20%	37.90%	34.60%	38.50%