Employee Benefit Plans - Stock Options (Details) - Share-based Payment Arrangement, Option [Member] - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Stock Options
Outstanding at beginning of year, shares	823,486	1,115,776	2,224,415
Exercised, shares	(603,823)	(292,290)	(1,108,639)
Canceled or expired, shares	0	0	0
Outstanding at end of year, shares	219,663	823,486	1,115,776
Exercisable at end of year, shares	219,663	823,486	1,115,776
Weighted Average Exercise Price
Outstanding at beginning of year, weighted average exercise price, per share	$ 23.84	$ 23.84	$ 19.94
Exercised, weighted average exercise price, per share	23.83	23.83	16.02
Canceled or expired, weighted average exercise price, per share	0	0	0
Outstanding at end of year, weighted average exercise price, per share	23.86	23.84	23.84
Exercisable at end of year, weighted average exercise price, per share	$ 23.86	$ 23.84	$ 23.84
Additional information - stock options
Expiration period	10 years
Total intrinsic value of options exercised	$ 47.2	$ 23.0	$ 82.5
Aggregate intrinsic value of options outstanding	$ 18.4
Lower range of options outstanding exercise price	$ 23.57
Upper range of options outstanding exercise price	$ 23.86
Weighted average remaining contractual term of options outstanding	4 months 24 days