Loans - Summary of Nonaccrual Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 322,146	$ 105,090
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	73,146	14,973
Agriculture
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	217,642	77,880
Commercial non-real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	26,843	9,502
Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	4,441	2,661
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 74	$ 74