SECURITIES - Schedule of Securities Available (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Available For Sale
Mortgage-back securities	$ 364,065	$ 586,454
Available-for-sale securities [Abstract]
Amortized cost	1,895,802	1,680,307
Gross unrealized gains	4,096	19,884
Gross unrealized (losses)	(47,873)	(6,760)
Total securities	1,852,025	1,693,431
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	950,499	518,797
OVER 12 MONTHS, Fair Value	601,825	106,140
TOTAL, Fair Value	1,552,324	624,937
LESS THAN 12 MONTHS, Unrealized (Losses)	(18,629)	(4,512)
OVER 12 MONTHS, Unrealized (Losses)	(29,244)	(2,248)
TOTAL, Unrealized (Losses)	(47,873)	(6,760)
AMORTIZED COST [Abstract]
Due in one year or less	2,532	0
Due after one year through five years	41,415	36,586
Due after five years through ten years	352,099	347,831
Due after ten years	1,118,283	705,963
Total Amortized Cost	1,514,329	1,090,380
Mortgage-backed securities	378,301	588,918
Common equities and mutual funds	3,172	1,009
Amortized cost	1,895,802	1,680,307
FAIR VALUE [Abstract]
Due in one year or less	2,529	0
Due after one year through five years	41,504	37,674
Due after five years through ten years	350,143	358,198
Due after ten years	1,089,984	709,660
Total Fair Value	1,484,160	1,105,532
Mortgage-back securities	364,065	586,454
Common equities and mutual funds	3,800	1,445
Total securities	1,852,025	1,693,431
Held To Maturity
Amortized cost	172,154	563,529
Gross unrealized gains		4,901
Gross unrealized (losses)	(11,180)	(4,245)
Fair value	160,974	564,185
Held-to-maturity securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	6,178	315,838
OVER 12 MONTHS, Fair Value	154,796	10,295
TOTAL, Fair Value	160,974	326,133
LESS THAN 12 MONTHS, Unrealized (Losses)	(287)	(4,022)
OVER 12 MONTHS, Unrealized (Losses)	(10,893)	(223)
TOTAL, Unrealized (Losses)	(11,180)	(4,245)
AMORTIZED COST [Abstract]
Due in one year or less		1,483
Due after one year through five years		17,926
Due after five years through ten years		144,996
Due after ten years	164,304	285,435
Total Amortized Cost	164,304	449,840
Mortgage-backed securities	7,850	113,689
Amortized cost	172,154	563,529
FAIR VALUE [Abstract]
Due in one year or less		1,480
Due after one year through five years		18,160
Due after five years through ten years		147,832
Due after ten years	153,546	284,257
Total Fair Value	153,546	451,729
Mortgage-backed securities	7,428	112,456
Total securities	160,974	564,185
Small Business Administration Securities [Member]
Available For Sale
Amortized cost	45,591	57,046
Gross unrealized gains	1	825
Gross unrealized (losses)	(1,255)	0
Mortgage-back securities	44,337	57,871
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	43,097
OVER 12 MONTHS, Fair Value	0
TOTAL, Fair Value	43,097
LESS THAN 12 MONTHS, Unrealized (Losses)	(1,255)
OVER 12 MONTHS, Unrealized (Losses)	0
TOTAL, Unrealized (Losses)	(1,255)
FAIR VALUE [Abstract]
Mortgage-back securities	44,337	57,871
US States and Political Subdivisions Debt Securities [Member]
Available For Sale
Amortized cost	17,154
Gross unrealized gains	49
Gross unrealized (losses)	(293)
Mortgage-back securities	16,910
Available-for-sale securities in a continuous unrealized loss position [Abstract]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	11,036
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(279)
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	881
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(14)
Debt Securities, Available-for-sale, Unrealized Loss Position	11,917
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss	(293)
FAIR VALUE [Abstract]
Mortgage-back securities	16,910
Held To Maturity
Amortized cost		19,247
Gross unrealized gains		157
Gross unrealized (losses)		(36)
Fair value		19,368
Held-to-maturity securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value		1,364
OVER 12 MONTHS, Fair Value		4,089
TOTAL, Fair Value		5,453
LESS THAN 12 MONTHS, Unrealized (Losses)		(6)
OVER 12 MONTHS, Unrealized (Losses)		(30)
TOTAL, Unrealized (Losses)		(36)
AMORTIZED COST [Abstract]
Amortized cost		19,247
FAIR VALUE [Abstract]
Total securities		19,368
Non-Bank Qualified Obligation of States And Political Subdivisions [Member]
Available For Sale
Amortized cost	1,140,884	938,883
Gross unrealized gains	826	14,983
Gross unrealized (losses)	(31,825)	(3,037)
Mortgage-back securities	1,109,885	950,829
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	626,693	280,900
OVER 12 MONTHS, Fair Value	358,095	5,853
TOTAL, Fair Value	984,788	286,753
LESS THAN 12 MONTHS, Unrealized (Losses)	(13,539)	(2,887)
OVER 12 MONTHS, Unrealized (Losses)	(18,286)	(150)
TOTAL, Unrealized (Losses)	(31,825)	(3,037)
FAIR VALUE [Abstract]
Mortgage-back securities	1,109,885	950,829
Held To Maturity
Amortized cost	164,304	430,593
Gross unrealized gains	0	4,744
Gross unrealized (losses)	(10,758)	(2,976)
Fair value	153,546	432,361
Held-to-maturity securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	6,178	202,018
OVER 12 MONTHS, Fair Value	147,368	6,206
TOTAL, Fair Value	153,546	208,224
LESS THAN 12 MONTHS, Unrealized (Losses)	(287)	(2,783)
OVER 12 MONTHS, Unrealized (Losses)	(10,471)	(193)
TOTAL, Unrealized (Losses)	(10,758)	(2,976)
AMORTIZED COST [Abstract]
Amortized cost	164,304	430,593
FAIR VALUE [Abstract]
Total securities	153,546	432,361
Asset-backed Securities [Member]
Available For Sale
Amortized cost	310,700	94,451
Gross unrealized gains	2,585	2,381
Gross unrealized (losses)	(257)	0
Mortgage-back securities	313,028	96,832
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	146,638
OVER 12 MONTHS, Fair Value	0
TOTAL, Fair Value	146,638
LESS THAN 12 MONTHS, Unrealized (Losses)	(257)
OVER 12 MONTHS, Unrealized (Losses)	0
TOTAL, Unrealized (Losses)	(257)
FAIR VALUE [Abstract]
Mortgage-back securities	313,028	96,832
Collateralized Mortgage Backed Securities [Member]
Available For Sale
Amortized cost	378,301	588,918
Gross unrealized gains	0	1,259
Gross unrealized (losses)	(14,236)	(3,723)
Mortgage-back securities	364,065	586,454
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	121,217	237,897
OVER 12 MONTHS, Fair Value	242,849	100,287
TOTAL, Fair Value	364,066	338,184
LESS THAN 12 MONTHS, Unrealized (Losses)	(3,292)	(1,625)
OVER 12 MONTHS, Unrealized (Losses)	(10,944)	(2,098)
TOTAL, Unrealized (Losses)	(14,236)	(3,723)
FAIR VALUE [Abstract]
Mortgage-back securities	364,065	586,454
Held To Maturity
Amortized cost	7,850	113,689
Gross unrealized gains	0	0
Gross unrealized (losses)	(422)	(1,233)
Fair value	7,428	112,456
Held-to-maturity securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	0	112,456
OVER 12 MONTHS, Fair Value	7,428	0
TOTAL, Fair Value	7,428	112,456
LESS THAN 12 MONTHS, Unrealized (Losses)	0	(1,233)
OVER 12 MONTHS, Unrealized (Losses)	(422)	0
TOTAL, Unrealized (Losses)	(422)	(1,233)
AMORTIZED COST [Abstract]
Amortized cost	7,850	113,689
FAIR VALUE [Abstract]
Total securities	7,428	112,456
Debt Securities [Member]
Available For Sale
Amortized cost	1,892,630	1,679,298
Gross unrealized gains	3,461	19,448
Gross unrealized (losses)	(47,866)	(6,760)
Mortgage-back securities	1,848,225	1,691,986
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	948,681
OVER 12 MONTHS, Fair Value	601,825
TOTAL, Fair Value	1,550,506
LESS THAN 12 MONTHS, Unrealized (Losses)	(18,622)
OVER 12 MONTHS, Unrealized (Losses)	(29,244)
TOTAL, Unrealized (Losses)	(47,866)
FAIR VALUE [Abstract]
Mortgage-back securities	1,848,225	1,691,986
Common Equities And Mutual Funds [Member]
Available For Sale
Amortized cost	3,172	1,009
Gross unrealized gains	635	436
Gross unrealized (losses)	(7)	0
Mortgage-back securities	3,800	1,445
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	1,818
OVER 12 MONTHS, Fair Value	0
TOTAL, Fair Value	1,818
LESS THAN 12 MONTHS, Unrealized (Losses)	(7)
OVER 12 MONTHS, Unrealized (Losses)	0
TOTAL, Unrealized (Losses)	(7)
FAIR VALUE [Abstract]
Mortgage-back securities	$ 3,800	$ 1,445