SECURITIES - Available for Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018
Available-for-sale debt securities [Abstract]
Fair Value	$ 354,186	$ 364,065
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value		950,499
LESS THAN 12 MONTHS, Unrealized (Losses)		(18,629)
OVER 12 MONTHS, Fair Value		601,825
OVER 12 MONTHS, Unrealized (Losses)		(29,244)
TOTAL, Fair Value		1,552,324
TOTAL, Unrealized (Losses)		(47,873)
AMORTIZED COST
Due in one year or less	1,585	2,532
Due after one year through five years	25,283	41,415
Due after five years through ten years	250,025	352,099
Due after ten years	1,089,907	1,118,283
Total	1,366,800	1,514,329
Mortgage-backed securities	366,467	378,301
Common equities and mutual funds		3,172
Amortized cost	1,733,267	1,895,802
FAIR VALUE
Due in one year or less	1,581	2,529
Due after one year through five years	25,483	41,504
Due after five years through ten years	251,933	350,143
Due after ten years	1,061,873	1,089,984
Total	1,340,870	1,484,160
Mortgage-backed securities	354,186	364,065
Common equities and mutual funds(1)		3,800
Total available for sale securities	1,695,056	1,852,025
Available-for-sale Securities [Member]
Available-for-sale debt securities [Abstract]
Amortized cost		1,895,802
Gross unrealized gains		4,096
Gross unrealized (losses)		(47,873)
Fair Value		1,852,025
Small Business Administration Securities [Member]
Available-for-sale debt securities [Abstract]
Amortized cost	94,242	45,591
Gross unrealized gains	165	1
Gross unrealized (losses)	(351)	(1,255)
Fair Value	94,056	44,337
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	0	43,097
LESS THAN 12 MONTHS, Unrealized (Losses)	0	(1,255)
OVER 12 MONTHS, Fair Value	28,318	0
OVER 12 MONTHS, Unrealized (Losses)	(351)	0
TOTAL, Fair Value	28,318	43,097
TOTAL, Unrealized (Losses)	(351)	(1,255)
Obligations of States and Political Subdivisions [Member]
Available-for-sale debt securities [Abstract]
Amortized cost	15,023	17,154
Gross unrealized gains	73	49
Gross unrealized (losses)	(146)	(293)
Fair Value	14,950	16,910
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	2,089	11,036
LESS THAN 12 MONTHS, Unrealized (Losses)	(6)	(279)
OVER 12 MONTHS, Fair Value	7,198	881
OVER 12 MONTHS, Unrealized (Losses)	(140)	(14)
TOTAL, Fair Value	9,287	11,917
TOTAL, Unrealized (Losses)	(146)	(293)
Non-bank Qualified Obligations of States and Political Subdivisions [Member]
Available-for-sale debt securities [Abstract]
Amortized cost	970,288	1,140,884
Gross unrealized gains	2,787	826
Gross unrealized (losses)	(28,504)	(31,825)
Fair Value	944,571	1,109,885
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	112,208	626,693
LESS THAN 12 MONTHS, Unrealized (Losses)	(2,443)	(13,539)
OVER 12 MONTHS, Fair Value	525,831	358,095
OVER 12 MONTHS, Unrealized (Losses)	(26,061)	(18,286)
TOTAL, Fair Value	638,039	984,788
TOTAL, Unrealized (Losses)	(28,504)	(31,825)
Asset-backed Securities [Member]
Available-for-sale debt securities [Abstract]
Amortized cost	287,247	310,700
Gross unrealized gains	1,261	2,585
Gross unrealized (losses)	(1,215)	(257)
Fair Value	287,293	313,028
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	162,550	146,638
LESS THAN 12 MONTHS, Unrealized (Losses)	(1,215)	(257)
OVER 12 MONTHS, Fair Value	0	0
OVER 12 MONTHS, Unrealized (Losses)	0	0
TOTAL, Fair Value	162,550	146,638
TOTAL, Unrealized (Losses)	(1,215)	(257)
Mortgage-backed Securities [Member]
Available-for-sale debt securities [Abstract]
Amortized cost	366,467	378,301
Gross unrealized gains	157	0
Gross unrealized (losses)	(12,438)	(14,236)
Fair Value	354,186	364,065
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	4,063	121,217
LESS THAN 12 MONTHS, Unrealized (Losses)	(16)	(3,292)
OVER 12 MONTHS, Fair Value	321,021	242,849
OVER 12 MONTHS, Unrealized (Losses)	(12,422)	(10,944)
TOTAL, Fair Value	325,084	364,066
TOTAL, Unrealized (Losses)	(12,438)	(14,236)
Debt Securities [Member]
Available-for-sale debt securities [Abstract]
Amortized cost	1,733,267	1,892,630
Gross unrealized gains	4,443	3,461
Gross unrealized (losses)	(42,654)	(47,866)
Fair Value	1,695,056	1,848,225
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	280,910	948,681
LESS THAN 12 MONTHS, Unrealized (Losses)	(3,680)	(18,622)
OVER 12 MONTHS, Fair Value	882,368	601,825
OVER 12 MONTHS, Unrealized (Losses)	(38,974)	(29,244)
TOTAL, Fair Value	1,163,278	1,550,506
TOTAL, Unrealized (Losses)	(42,654)	(47,866)
Common Equities and Mutual Funds [Member]
Available-for-sale debt securities [Abstract]
Amortized cost	3,494	3,172
Gross unrealized gains	357	635
Gross unrealized (losses)	(14)	(7)
Fair Value	3,837	3,800
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	2,001	1,818
LESS THAN 12 MONTHS, Unrealized (Losses)	(14)	(7)
OVER 12 MONTHS, Fair Value	0	0
OVER 12 MONTHS, Unrealized (Losses)	0	0
TOTAL, Fair Value	2,001	1,818
TOTAL, Unrealized (Losses)	$ (14)	$ (7)