Loans - Schedule of Loans Receivable (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	$ 9,717,115	$ 9,732,684
Less: Unamortized discount on acquired loans	(10,468)	(13,655)
Unearned net deferred fees and costs and loans in process	(13,352)	(12,266)
Total	9,693,295	9,706,763
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	5,222,819	5,092,410
Agriculture
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	1,881,792	2,008,644
Commercial non-real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	1,699,197	1,719,956
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	820,759	812,208
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	52,640	51,925
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	$ 39,908	$ 47,541