Stock-Based Compensation Plans - Summary of Company's Share Awards (Details) - $ / shares	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Equity Units Weighted-Average Intrinsic Value
Granted (in usd per share)	$ 23.82	$ 3.13	$ 3.12
Deferred Equity Units
Equity Units
Unvested units beginning balance (in shares)	1,900,747	2,863,456
Granted (in shares)	0	0
Vested (in shares)	(1,351,293)	(962,709)
Forfeited (in shares)	0	0
Unvested units ending balance (in shares)	549,454	1,900,747	2,863,456
Equity Units Weighted-Average Intrinsic Value
Unvested units beginning balance (in usd per share)	$ 7.71	$ 6.37
Granted (in usd per share)	0	0
Vested (in usd per share)	27.01	7.71
Forfeited (in usd per share)	0	0
Unvested units ending balance (in usd per share)	27.01	7.71	$ 6.37
Equity Units Weighted-Average Grant-Date Intrinsic Value
Unvested units beginning balance (in usd per share)	3.13	3.12
Granted (in usd per share)	0	0
Vested (in usd per share)	23.91	3.09
Forfeited (in usd per share)	0	0
Unvested units ending balance (in usd per share)	$ 23.82	$ 3.13	$ 3.12
Matching Equity Units
Equity Units
Unvested units beginning balance (in shares)	6,681,397	12,885,551
Granted (in shares)	0	0
Vested (in shares)	(2,369,536)	(6,204,154)
Forfeited (in shares)	0	0
Unvested units ending balance (in shares)	4,311,861	6,681,397	12,885,551
Equity Units Weighted-Average Intrinsic Value
Unvested units beginning balance (in usd per share)	$ 4.60	$ 3.50
Granted (in usd per share)	0	0
Vested (in usd per share)	24.04	5.10
Forfeited (in usd per share)	0	0
Unvested units ending balance (in usd per share)	23.82	4.60	$ 3.50
Equity Units Weighted-Average Grant-Date Intrinsic Value
Unvested units beginning balance (in usd per share)	0	0
Granted (in usd per share)	0	0
Vested (in usd per share)	0	0
Forfeited (in usd per share)	0	0
Unvested units ending balance (in usd per share)	$ 0	$ 0	$ 0