Loans - Schedule of Impaired Loans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2019
With an allowance recorded:
Recorded Investment	$ 206,474		$ 206,474		$ 149,762
Unpaid Principal Balance	215,657		215,657		156,949
Related Allowance	41,807		41,807		20,347
With no allowance recorded:
Recorded Investment	426,874		426,874		284,229
Unpaid Principal Balance	495,096		495,096		358,915
Total Recorded Investment, Impaired Loans	633,348		633,348		433,991
Total Unpaid Principal Balance, Impaired Loans	710,753		710,753		515,864
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	609,205	$ 326,992	562,302	$ 267,548
Interest Income Recognized While on Impaired Status	5,905	8,464	27,670	13,234
Commercial real estate
With an allowance recorded:
Recorded Investment	84,064		84,064		26,003
Unpaid Principal Balance	85,941		85,941		26,297
Related Allowance	18,993		18,993		4,159
With no allowance recorded:
Recorded Investment	89,000		89,000		28,272
Unpaid Principal Balance	127,589		127,589		66,631
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	152,871	40,199	113,146	39,399
Interest Income Recognized While on Impaired Status	1,684	718	5,350	1,415
Agriculture
With an allowance recorded:
Recorded Investment	88,107		88,107		98,392
Unpaid Principal Balance	91,439		91,439		104,350
Related Allowance	10,835		10,835		8,234
With no allowance recorded:
Recorded Investment	260,026		260,026		231,087
Unpaid Principal Balance	278,906		278,906		255,308
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	348,006	253,240	354,202	196,563
Interest Income Recognized While on Impaired Status	3,421	7,147	16,722	10,349
Commercial non-real estate
With an allowance recorded:
Recorded Investment	28,684		28,684		21,331
Unpaid Principal Balance	32,127		32,127		21,777
Related Allowance	9,846		9,846		6,062
With no allowance recorded:
Recorded Investment	72,458		72,458		21,579
Unpaid Principal Balance	80,692		80,692		31,414
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	97,310	26,381	84,849	24,518
Interest Income Recognized While on Impaired Status	678	468	5,079	1,146
Residential real estate
With an allowance recorded:
Recorded Investment	5,513		5,513		3,829
Unpaid Principal Balance	6,034		6,034		4,311
Related Allowance	2,101		2,101		1,795
With no allowance recorded:
Recorded Investment	5,389		5,389		3,290
Unpaid Principal Balance	7,801		7,801		5,454
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	10,901	6,911	9,957	6,831
Interest Income Recognized While on Impaired Status	120	123	513	305
Consumer
With an allowance recorded:
Recorded Investment	106		106		207
Unpaid Principal Balance	116		116		214
Related Allowance	32		32		97
With no allowance recorded:
Recorded Investment	1		1		1
Unpaid Principal Balance	108		108		$ 108
Average recorded investment and interest income recognized on impaired loans:
Average Recorded Investment	117	261	148	237
Interest Income Recognized While on Impaired Status	$ 2	$ 8	$ 6	$ 19