Loans - Schedule of the Company's Nonaccrual Loans (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	$ 210,607	$ 105,090
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	41,541	14,973
Agriculture
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	143,198	77,880
Commercial non-real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	21,334	9,502
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	4,437	2,661
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	$ 97	$ 74