Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Deferred tax assets:
Allowance for loan and lease losses	$ 36,177	$ 17,214
Compensation	3,547	4,017
Securities available for sale	0	0
Other real estate owned	3,067	574
Core deposit intangible and other fair value adjustments	2,554	3,868
Excess tax basis of FDIC indemnification asset and clawback liability	0	4,095
Excess tax basis of loans acquired over carrying value	2,059	2,656
Other reserves	9,245	2,926
Goodwill and other intangibles	11,075	0
Lease liability	5,805	0
Other	7,268	6,350
Total deferred tax assets	80,797	41,700
Deferred tax liabilities:
Goodwill and other intangibles	0	(15,064)
Securities available for sale	(11,749)	(4,419)
Premises and equipment	(7,167)	(5,869)
Deferred REIT income	(8,522)	(8,783)
Right of use asset	(5,399)	0
Other	(251)	(279)
Total deferred tax liabilities	(33,088)	(34,414)
Net deferred tax assets	$ 47,709	$ 7,286