Summary of Significant Accounting Policies - Goodwill and Intangible Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022
Goodwill
Goodwill, beginning of period	$ 303,457	$ 303,457
Additions	1,293	0
Goodwill, end of period	304,750	303,457
Core Deposit and Other Intangibles
Intangible assets, beginning of period	5,552	6,562
Additions	1,297	0
Amortization	(980)	(1,010)
Intangible assets, end of period	5,869	5,552
Total
Goodwill and intangible assets, beginning of period	309,009	310,019
Additions	2,590	0
Amortization	(980)	(1,010)
Goodwill and intangible assets, end of period	$ 310,619	$ 309,009