Loans - Schedule of the Composition of the Loan Portfolio by Internal Risk Rating (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	$ 7,691,349	$ 7,491,586
Loans covered by FDIC loss share agreements	61,789	73,272
Total	7,753,138	7,564,858
Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	7,135,300	6,921,071
Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	298,963	327,608
Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	250,610	241,605
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	6,476	1,302
Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	880,646	934,980
Loans covered by FDIC loss share agreements	61,789	73,272
Total	942,435	1,008,252
Residential real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	867,159	919,224
Residential real estate | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	4,606	4,741
Residential real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	8,395	10,885
Residential real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	486	130
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	3,655,902	3,414,758
Loans covered by FDIC loss share agreements	0	0
Total	3,655,902	3,414,758
Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	3,548,040	3,276,048
Commercial real estate | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	63,743	81,148
Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	43,540	57,415
Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	579	147
Commercial non real estate
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,223,503	1,173,910
Loans covered by FDIC loss share agreements	0	0
Total	1,223,503	1,173,910
Commercial non real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,151,190	1,093,913
Commercial non real estate | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	36,847	37,283
Commercial non real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	30,184	42,319
Commercial non real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	5,282	395
Agriculture
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,817,619	1,849,869
Loans covered by FDIC loss share agreements	0	0
Total	1,817,619	1,849,869
Agriculture | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,455,712	1,514,344
Agriculture | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	193,703	204,326
Agriculture | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	168,075	130,569
Agriculture | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	129	630
Consumer
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	69,568	75,592
Loans covered by FDIC loss share agreements	0	0
Total	69,568	75,592
Consumer | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	69,088	75,065
Consumer | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	64	110
Consumer | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	416	417
Consumer | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Other
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	44,111	42,477
Loans covered by FDIC loss share agreements	0	0
Total	44,111	42,477
Other | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	44,111	42,477
Other | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Other | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	0	0
Other | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	$ 0	$ 0