Loans - Schedule of the Company's Nonaccrual Loans (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	$ 271,874	$ 105,090
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	96,765	14,973
Agriculture
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	150,009	77,880
Commercial non-real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	20,414	9,502
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	4,607	2,661
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	$ 79	$ 74