Loans - Schedule of Loans Receivable (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	$ 8,830,042	$ 8,735,918
Less: Unamortized discount on acquired loans	(37,304)	(39,947)
Unearned net deferred fees and costs and loans in process	(13,631)	(13,327)
Net loans	8,779,107	8,682,644
Residential real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	1,008,325	1,020,958
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	3,852,104	3,754,107
Commercial non real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	1,643,986	1,673,166
Agriculture
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	2,206,263	2,168,937
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	71,795	76,273
Other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total financing receivables, gross	$ 47,569	$ 42,477