Loans - Summary of Nonaccrual Loans (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 140,566	$ 132,225
Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	22,871	14,693
Agriculture
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	107,198	99,325
Commercial non-real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	6,887	13,674
Residential real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	3,549	4,421
Consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Nonaccrual loans	$ 61	$ 112