Loans - Summary of Troubled Debt Restructurings on Accruing and Nonaccrual Financing Receivables (Details) $ in Thousands	3 Months Ended
	Dec. 31, 2016 USD ($) contract	Dec. 31, 2015 USD ($) contract	Sep. 30, 2016 USD ($)
Accruing
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 41,854		$ 46,568
Financing receivable, modifications, number of contracts | contract	3	11
Financing receivable, modifications, pre-modification recorded investment	$ 442	$ 23,929
Financing receivable, modifications, post-modification recorded investment	$ 442	$ 23,929
Change in recorded investment due to principal paydown at time of modification, number of contracts | contract	0	0
Change in recorded investment due to principal paydown at time of modification, pre-modification	$ 0	$ 0
Change in recorded investment due to principal paydown at time of modification, post-modification	$ 0	$ 0
Change in recorded investment due to chargeoffs at time of modification, number of contracts | contract	0	0
Change in recorded investment due to chargeoffs at time of modification, pre-modification	$ 0	$ 0
Change in recorded investment due to chargeoffs at time of modification, post-modification	0	$ 0
Accruing | Residential real estate
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 369		370
Financing receivable, modifications, number of contracts | contract	1	0
Financing receivable, modifications, pre-modification recorded investment	$ 9	$ 0
Financing receivable, modifications, post-modification recorded investment	9	$ 0
Accruing | Commercial real estate
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 14,439		18,250
Financing receivable, modifications, number of contracts | contract	0	2
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 1,898
Financing receivable, modifications, post-modification recorded investment	0	$ 1,898
Accruing | Commercial non real estate
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 7,250		8,102
Financing receivable, modifications, number of contracts | contract	2	1
Financing receivable, modifications, pre-modification recorded investment	$ 433	$ 58
Financing receivable, modifications, post-modification recorded investment	433	$ 58
Accruing | Agriculture
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 19,789		19,823
Financing receivable, modifications, number of contracts | contract	0	8
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 21,973
Financing receivable, modifications, post-modification recorded investment	0	$ 21,973
Accruing | Consumer
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 7		23
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Rate modification | Residential real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Rate modification | Commercial real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Rate modification | Commercial non real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Rate modification | Agriculture
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Rate modification | Consumer
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Term extension | Residential real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Term extension | Commercial real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	2
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 1,898
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 1,898
Accruing | Term extension | Commercial non real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	1
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 58
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 58
Accruing | Term extension | Agriculture
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	8
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 21,973
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 21,973
Accruing | Term extension | Consumer
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Payment modification | Residential real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	1	0
Financing receivable, modifications, pre-modification recorded investment	$ 9	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 9	$ 0
Accruing | Payment modification | Commercial real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Payment modification | Commercial non real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	2	0
Financing receivable, modifications, pre-modification recorded investment	$ 433	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 433	$ 0
Accruing | Payment modification | Agriculture
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Payment modification | Consumer
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Bankruptcy | Residential real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Bankruptcy | Commercial real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Bankruptcy | Commercial non real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Bankruptcy | Agriculture
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Bankruptcy | Consumer
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Other | Residential real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Other | Commercial real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Other | Commercial non real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Other | Agriculture
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Accruing | Other | Consumer
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	0	$ 0
Nonaccrual
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 33,960		36,778
Financing receivable, modifications, number of contracts | contract	1	2
Financing receivable, modifications, pre-modification recorded investment	$ 21	$ 583
Financing receivable, modifications, post-modification recorded investment	$ 21	$ 583
Change in recorded investment due to principal paydown at time of modification, number of contracts | contract	0	0
Change in recorded investment due to principal paydown at time of modification, pre-modification	$ 0	$ 0
Change in recorded investment due to principal paydown at time of modification, post-modification	$ 0	$ 0
Change in recorded investment due to chargeoffs at time of modification, number of contracts | contract	0	0
Change in recorded investment due to chargeoffs at time of modification, pre-modification	$ 0	$ 0
Change in recorded investment due to chargeoffs at time of modification, post-modification	0	$ 0
Nonaccrual | Residential real estate
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 910		937
Financing receivable, modifications, number of contracts | contract	1	1
Financing receivable, modifications, pre-modification recorded investment	$ 21	$ 187
Financing receivable, modifications, post-modification recorded investment	21	$ 187
Nonaccrual | Commercial real estate
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 2,186		2,356
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	0	$ 0
Nonaccrual | Commercial non real estate
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 2,624		4,789
Financing receivable, modifications, number of contracts | contract	0	1
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 396
Financing receivable, modifications, post-modification recorded investment	0	$ 396
Nonaccrual | Agriculture
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 28,232		28,688
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	0	$ 0
Nonaccrual | Consumer
Financing Receivable, Modifications [Line Items]
Recorded value of TDR balance	$ 8		$ 8
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Rate modification | Residential real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Rate modification | Commercial real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Rate modification | Commercial non real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Rate modification | Agriculture
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Rate modification | Consumer
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Term extension | Residential real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Term extension | Commercial real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Term extension | Commercial non real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Term extension | Agriculture
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Term extension | Consumer
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Payment modification | Residential real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	1	1
Financing receivable, modifications, pre-modification recorded investment	$ 21	$ 187
Financing receivable, modifications, post-modification recorded investment	$ 21	$ 187
Nonaccrual | Payment modification | Commercial real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Payment modification | Commercial non real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	1
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 396
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 396
Nonaccrual | Payment modification | Agriculture
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Payment modification | Consumer
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Bankruptcy | Residential real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Bankruptcy | Commercial real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Bankruptcy | Commercial non real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Bankruptcy | Agriculture
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Bankruptcy | Consumer
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Other | Residential real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Other | Commercial real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Other | Commercial non real estate
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Other | Agriculture
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0
Nonaccrual | Other | Consumer
Financing Receivable, Modifications [Line Items]
Financing receivable, modifications, number of contracts | contract	0	0
Financing receivable, modifications, pre-modification recorded investment	$ 0	$ 0
Financing receivable, modifications, post-modification recorded investment	$ 0	$ 0