Loans - Impaired Financing Receivables (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
With an allowance recorded:
Recorded Investment	$ 104,035	$ 123,445
Unpaid Principal Balance	124,656	148,077
Related Allowance	20,037	22,535
With no allowance recorded:
Recorded Investment	104,019	84,478
Unpaid Principal Balance	161,235	183,341
Total Recorded Investment, Impaired Loans	208,054	207,923
Total Unpaid Principal Balance, Impaired Loans	285,891	331,418
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	218,230	224,131	$ 227,992
Interest Income Recognized While on Impaired Status	9,453	9,651	12,980
Commercial real estate
With an allowance recorded:
Recorded Investment	25,136	20,819
Unpaid Principal Balance	25,223	24,893
Related Allowance	3,668	3,621
With no allowance recorded:
Recorded Investment	15,764	16,652
Unpaid Principal Balance	58,141	69,677
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	54,434	42,347	70,266
Interest Income Recognized While on Impaired Status	2,815	2,163	3,876
Agriculture
With an allowance recorded:
Recorded Investment	60,053	79,219
Unpaid Principal Balance	76,874	88,268
Related Allowance	9,590	11,468
With no allowance recorded:
Recorded Investment	77,172	51,256
Unpaid Principal Balance	80,355	64,177
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	127,483	131,026	100,052
Interest Income Recognized While on Impaired Status	4,767	5,503	6,502
Commercial non-real estate
With an allowance recorded:
Recorded Investment	14,177	17,950
Unpaid Principal Balance	17,241	28,755
Related Allowance	4,508	4,779
With no allowance recorded:
Recorded Investment	8,905	13,983
Unpaid Principal Balance	18,047	38,924
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	28,938	41,489	45,592
Interest Income Recognized While on Impaired Status	1,405	1,485	1,971
Residential real estate
With an allowance recorded:
Recorded Investment	4,509	5,177
Unpaid Principal Balance	5,153	5,874
Related Allowance	2,210	2,581
With no allowance recorded:
Recorded Investment	2,177	2,574
Unpaid Principal Balance	4,574	9,613
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	7,156	8,900	11,773
Interest Income Recognized While on Impaired Status	452	453	576
Consumer
With an allowance recorded:
Recorded Investment	160	280
Unpaid Principal Balance	165	287
Related Allowance	61	86
With no allowance recorded:
Recorded Investment	1	13
Unpaid Principal Balance	118	950
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	219	369	309
Interest Income Recognized While on Impaired Status	$ 14	$ 47	$ 55