SECURITIES - Schedule of Securities Available (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018
Available For Sale
Amortized cost	$ 1,263,888	$ 1,895,802
Gross unrealized gains		4,096
Gross unrealized (losses)		(47,873)
Total securities	1,272,493	1,852,025
Available-for-sale securities in a continuous unrealized loss position [Abstract]
Fair Value, Less than 12 months		950,499
Unrealized (Losses), Less than 12 months		(18,629)
Fair Value, 12 Months or Longer		601,825
Unrealized (Losses), 12 Months or Longer		(29,244)
Fair Value, Total		1,552,324
Unrealized (Losses), Total		(47,873)
AMORTIZED COST [Abstract]
Due in one year or less	0	2,532
Due after one year through five years	16,749	41,415
Due after five years through ten years	50,263	352,099
Due after ten years	818,206	1,118,283
Total Amortized Cost	885,218	1,514,329
Mortgage-backed securities	378,670	378,301
Common equities and mutual funds		3,172
Amortized cost	1,263,888	1,895,802
FAIR VALUE [Abstract]
Due in one year or less	0	2,529
Due after one year through five years	17,143	41,504
Due after five years through ten years	51,840	350,143
Due after ten years	820,964	1,089,984
Total Fair Value	889,947	1,484,160
Mortgage-back securities	382,546	364,065
Common equities and mutual funds		3,800
Total securities	1,272,493	1,852,025
Held To Maturity
Amortized cost	134,764	171,743
Gross unrealized gains	122	0
Gross unrealized (losses)	(1,416)	(11,180)
Fair value	133,470	160,563
Held-to-maturity securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	7,438	5,767
OVER 12 MONTHS, Fair Value	111,171	154,796
TOTAL, Fair Value	118,609	160,563
LESS THAN 12 MONTHS, Unrealized (Losses)	(6)	(287)
OVER 12 MONTHS, Unrealized (Losses)	(1,410)	(10,893)
TOTAL, Unrealized (Losses)	(1,416)	(11,180)
AMORTIZED COST [Abstract]
Due after ten years	127,582	163,893
Total Amortized Cost	127,582	163,893
Mortgage-backed securities	7,182	7,850
Amortized cost	134,764	171,743
FAIR VALUE [Abstract]
Due after ten years	126,287	153,135
Total Fair Value	126,287	153,135
Mortgage-backed securities	7,183	7,428
Total securities	133,470	160,563
Small Business Administration Securities
Available For Sale
Amortized cost	182,327	45,591
Gross unrealized gains	3,655	1
Gross unrealized (losses)	0	(1,255)
Total securities	185,982	44,337
Available-for-sale securities in a continuous unrealized loss position [Abstract]
Fair Value, Less than 12 months	10,262	43,097
Unrealized (Losses), Less than 12 months	0	(1,255)
Fair Value, 12 Months or Longer	0	0
Unrealized (Losses), 12 Months or Longer	0	0
Fair Value, Total	10,262	43,097
Unrealized (Losses), Total	0	(1,255)
AMORTIZED COST [Abstract]
Amortized cost	182,327	45,591
FAIR VALUE [Abstract]
Total securities	185,982	44,337
US States and Political Subdivisions Debt Securities
Available For Sale
Amortized cost	858	17,154
Gross unrealized gains	16	49
Gross unrealized (losses)	0	(293)
Total securities	874	16,910
Available-for-sale securities in a continuous unrealized loss position [Abstract]
Fair Value, Less than 12 months	0	11,036
Unrealized (Losses), Less than 12 months	0	(279)
Fair Value, 12 Months or Longer	0	881
Unrealized (Losses), 12 Months or Longer	0	(14)
Fair Value, Total	0	11,917
Unrealized (Losses), Total	0	(293)
AMORTIZED COST [Abstract]
Amortized cost	858	17,154
FAIR VALUE [Abstract]
Total securities	874	16,910
Non-Bank Qualified Obligation of States And Political Subdivisions
Available For Sale
Amortized cost	396,430	1,140,884
Gross unrealized gains	5,030	826
Gross unrealized (losses)	(903)	(31,825)
Total securities	400,557	1,109,885
Available-for-sale securities in a continuous unrealized loss position [Abstract]
Fair Value, Less than 12 months	66,326	626,693
Unrealized (Losses), Less than 12 months	(177)	(13,539)
Fair Value, 12 Months or Longer	55,428	358,095
Unrealized (Losses), 12 Months or Longer	(726)	(18,286)
Fair Value, Total	121,754	984,788
Unrealized (Losses), Total	(903)	(31,825)
AMORTIZED COST [Abstract]
Amortized cost	396,430	1,140,884
FAIR VALUE [Abstract]
Total securities	400,557	1,109,885
Held To Maturity
Amortized cost	127,582	163,893
Gross unrealized gains	108	0
Gross unrealized (losses)	(1,403)	(10,758)
Fair value	126,287	153,135
Held-to-maturity securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	5,967	5,767
OVER 12 MONTHS, Fair Value	109,368	147,368
TOTAL, Fair Value	115,335	153,135
LESS THAN 12 MONTHS, Unrealized (Losses)	(6)	(287)
OVER 12 MONTHS, Unrealized (Losses)	(1,397)	(10,471)
TOTAL, Unrealized (Losses)	(1,403)	(10,758)
AMORTIZED COST [Abstract]
Amortized cost	127,582	163,893
FAIR VALUE [Abstract]
Total securities	126,287	153,135
Asset-backed Securities
Available For Sale
Amortized cost	305,603	310,700
Gross unrealized gains	262	2,585
Gross unrealized (losses)	(3,331)	(257)
Total securities	302,534	313,028
Available-for-sale securities in a continuous unrealized loss position [Abstract]
Fair Value, Less than 12 months	158,176	146,638
Unrealized (Losses), Less than 12 months	(1,823)	(257)
Fair Value, 12 Months or Longer	93,259	0
Unrealized (Losses), 12 Months or Longer	(1,508)	0
Fair Value, Total	251,435	146,638
Unrealized (Losses), Total	(3,331)	(257)
AMORTIZED COST [Abstract]
Amortized cost	305,603	310,700
FAIR VALUE [Abstract]
Total securities	302,534	313,028
Collateralized Mortgage Backed Securities
Available For Sale
Amortized cost	378,670	378,301
Gross unrealized gains	5,731	0
Gross unrealized (losses)	(1,855)	(14,236)
Total securities	382,546	364,065
Available-for-sale securities in a continuous unrealized loss position [Abstract]
Fair Value, Less than 12 months	1,713	121,217
Unrealized (Losses), Less than 12 months	(1)	(3,292)
Fair Value, 12 Months or Longer	89,634	242,849
Unrealized (Losses), 12 Months or Longer	(1,854)	(10,944)
Fair Value, Total	91,347	364,066
Unrealized (Losses), Total	(1,855)	(14,236)
AMORTIZED COST [Abstract]
Amortized cost	378,670	378,301
FAIR VALUE [Abstract]
Total securities	382,546	364,065
Held To Maturity
Amortized cost	7,182	7,850
Gross unrealized gains	14	0
Gross unrealized (losses)	(13)	(422)
Fair value	7,183	7,428
Held-to-maturity securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	1,471	0
OVER 12 MONTHS, Fair Value	1,803	7,428
TOTAL, Fair Value	3,274	7,428
LESS THAN 12 MONTHS, Unrealized (Losses)	0	0
OVER 12 MONTHS, Unrealized (Losses)	(13)	(422)
TOTAL, Unrealized (Losses)	(13)	(422)
AMORTIZED COST [Abstract]
Amortized cost	7,182	7,850
FAIR VALUE [Abstract]
Total securities	7,183	7,428
Debt Securities
Available For Sale
Amortized cost	1,263,888	1,892,630
Gross unrealized gains	14,694	3,461
Gross unrealized (losses)	(6,089)	(47,866)
Total securities	1,272,493	1,848,225
Available-for-sale securities in a continuous unrealized loss position [Abstract]
Fair Value, Less than 12 months	236,477	948,681
Unrealized (Losses), Less than 12 months	(2,001)	(18,622)
Fair Value, 12 Months or Longer	238,321	601,825
Unrealized (Losses), 12 Months or Longer	(4,088)	(29,244)
Fair Value, Total	474,798	1,550,506
Unrealized (Losses), Total	(6,089)	(47,866)
AMORTIZED COST [Abstract]
Amortized cost	1,263,888	1,892,630
FAIR VALUE [Abstract]
Total securities	1,272,493	1,848,225
Common Equities And Mutual Funds
Available For Sale
Amortized cost	2,435	3,172
Gross unrealized gains	171	635
Gross unrealized (losses)	0	(7)
Total securities	2,606	3,800
Available-for-sale securities in a continuous unrealized loss position [Abstract]
Fair Value, Less than 12 months	0	1,818
Unrealized (Losses), Less than 12 months	0	(7)
Fair Value, 12 Months or Longer	0	0
Unrealized (Losses), 12 Months or Longer	0	0
Fair Value, Total	0	1,818
Unrealized (Losses), Total	0	(7)
AMORTIZED COST [Abstract]
Amortized cost	2,435	3,172
FAIR VALUE [Abstract]
Total securities	$ 2,606	$ 3,800