Loans - Schedule of the Company's Nonaccrual Loans (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	$ 112,426	$ 113,636
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	5,602	5,962
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	13,399	13,870
Commercial non real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	28,203	27,280
Agriculture
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	65,032	66,301
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual loans, excluding loans covered under FDIC loss-sharing arrangements	$ 190	$ 223