Loans - Schedule of the Composition of the Loan Portfolio by Internal Risk Rating (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	$ 7,645,722	$ 7,491,586
Loans covered by FDIC loss share agreements	68,553	73,272
Total	7,714,275	7,564,858
Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	7,060,711	6,921,071
Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	334,673	327,608
Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	249,518	241,605
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	820	1,302
Residential real estate
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	927,594	934,980
Loans covered by FDIC loss share agreements	68,553	73,272
Total	996,147	1,008,252
Residential real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	912,174	919,224
Residential real estate | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	5,175	4,741
Residential real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	10,150	10,885
Residential real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	95	130
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	3,551,434	3,414,758
Loans covered by FDIC loss share agreements	0	0
Total	3,551,434	3,414,758
Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	3,432,849	3,276,048
Commercial real estate | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	66,334	81,148
Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	52,111	57,415
Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	140	147
Commercial non real estate
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,138,286	1,173,910
Loans covered by FDIC loss share agreements	0	0
Total	1,138,286	1,173,910
Commercial non real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,056,226	1,093,913
Commercial non real estate | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	40,372	37,283
Commercial non real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	41,328	42,319
Commercial non real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	360	395
Agriculture
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,909,645	1,849,869
Loans covered by FDIC loss share agreements	0	0
Total	1,909,645	1,849,869
Agriculture | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	1,541,184	1,514,344
Agriculture | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	222,682	204,326
Agriculture | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	145,573	130,569
Agriculture | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	206	630
Consumer
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	71,194	75,592
Loans covered by FDIC loss share agreements	0	0
Total	71,194	75,592
Consumer | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	70,709	75,065
Consumer | Watchlist
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	110	110
Consumer | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	356	417
Consumer | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	19	0
Other
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	47,569	42,477
Loans covered by FDIC loss share agreements	0	0
Total	47,569	42,477
Other | Pass
Financing Receivable, Recorded Investment [Line Items]
Total financing receivables	47,569	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