Loans - Impaired Financing Receivables (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
With an allowance recorded:
Recorded Investment	$ 209,762	$ 149,762
Unpaid Principal Balance	223,173	156,949
Related Allowance	42,993	20,347
With no allowance recorded:
Recorded Investment	503,199	284,229
Unpaid Principal Balance	577,774	358,915
Total Recorded Investment, Impaired Loans	712,961	433,991
Total Unpaid Principal Balance, Impaired Loans	800,947	515,864
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	592,434	300,836	$ 218,230
Interest Income Recognized While on Impaired Status	39,900	17,653	9,453
Commercial real estate
With an allowance recorded:
Recorded Investment	111,121	26,003
Unpaid Principal Balance	114,034	26,297
Related Allowance	25,087	4,159
With no allowance recorded:
Recorded Investment	121,380	28,272
Unpaid Principal Balance	161,211	66,631
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	137,017	42,374	54,434
Interest Income Recognized While on Impaired Status	8,528	2,339	2,815
Agriculture
With an allowance recorded:
Recorded Investment	53,052	98,392
Unpaid Principal Balance	55,145	104,350
Related Allowance	8,151	8,234
With no allowance recorded:
Recorded Investment	308,734	231,087
Unpaid Principal Balance	332,272	255,308
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	355,719	223,146	127,483
Interest Income Recognized While on Impaired Status	22,927	13,093	4,767
Commercial non-real estate
With an allowance recorded:
Recorded Investment	39,821	21,331
Unpaid Principal Balance	47,571	21,777
Related Allowance	7,822	6,062
With no allowance recorded:
Recorded Investment	66,542	21,579
Unpaid Principal Balance	75,365	31,414
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	89,152	28,196	28,938
Interest Income Recognized While on Impaired Status	7,745	1,791	1,405
Residential real estate
With an allowance recorded:
Recorded Investment	5,670	3,829
Unpaid Principal Balance	6,314	4,311
Related Allowance	1,903	1,795
With no allowance recorded:
Recorded Investment	6,543	3,290
Unpaid Principal Balance	8,818	5,454
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	10,408	6,889	7,156
Interest Income Recognized While on Impaired Status	691	410	452
Consumer
With an allowance recorded:
Recorded Investment	98	207
Unpaid Principal Balance	109	214
Related Allowance	30	97
With no allowance recorded:
Recorded Investment	0	1
Unpaid Principal Balance	108	108
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	138	231	219
Interest Income Recognized While on Impaired Status	$ 9	$ 20	$ 14