Loans - Schedule of Loans Receivable (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	$ 10,356,359	$ 9,732,684
Less: Unamortized discount on acquired loans	(9,098)	(13,655)
Unearned net deferred fees and costs and loans in process	(33,262)	(12,266)
Total	10,313,999	9,706,763
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	5,355,493	5,092,410
Agriculture
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	1,815,121	2,008,644
Commercial non-real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	2,226,759	1,719,956
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	862,821	812,208
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	61,452	51,925
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	$ 34,713	$ 47,541