SECURITIES - Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Sep. 30, 2018
Available-for-sale debt securities [Abstract]
Fair Value	$ 413,493	$ 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	1,020	2,532
Due after one year through five years	15,058	41,415
Due after five years through ten years	64,894	352,099
Due after ten years	1,008,514	1,118,283
Total	1,089,486	1,514,329
Mortgage-backed securities	419,024	378,301
Common equities and mutual funds		3,172
Amortized cost	1,508,510	1,895,802
FAIR VALUE
Due in one year or less	1,019	2,529
Due after one year through five years	15,285	41,504
Due after five years through ten years	65,335	350,143
Due after ten years	1,000,024	1,089,984
Total	1,081,663	1,484,160
Mortgage-backed securities	413,493	364,065
Common equities and mutual funds(1)		3,800
Total available for sale securities	1,495,156	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
Available-for-sale debt securities [Abstract]
Amortized cost	163,621	45,591
Gross unrealized gains	637	1
Gross unrealized (losses)	(155)	(1,255)
Fair Value	164,103	44,337
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	41,845	43,097
LESS THAN 12 MONTHS, Unrealized (Losses)	(29)	(1,255)
OVER 12 MONTHS, Fair Value	27,214	0
OVER 12 MONTHS, Unrealized (Losses)	(126)	0
TOTAL, Fair Value	69,059	43,097
TOTAL, Unrealized (Losses)	(155)	(1,255)
Obligations of States and Political Subdivisions
Available-for-sale debt securities [Abstract]
Amortized cost	11,432	17,154
Gross unrealized gains	106	49
Gross unrealized (losses)	(9)	(293)
Fair Value	11,529	16,910
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	0	11,036
LESS THAN 12 MONTHS, Unrealized (Losses)	0	(279)
OVER 12 MONTHS, Fair Value	2,052	881
OVER 12 MONTHS, Unrealized (Losses)	(9)	(14)
TOTAL, Fair Value	2,052	11,917
TOTAL, Unrealized (Losses)	(9)	(293)
Non-bank Qualified Obligations of States and Political Subdivisions
Available-for-sale debt securities [Abstract]
Amortized cost	629,732	1,140,884
Gross unrealized gains	1,357	826
Gross unrealized (losses)	(9,447)	(31,825)
Fair Value	621,642	1,109,885
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	10,210	626,693
LESS THAN 12 MONTHS, Unrealized (Losses)	(2)	(13,539)
OVER 12 MONTHS, Fair Value	448,680	358,095
OVER 12 MONTHS, Unrealized (Losses)	(9,445)	(18,286)
TOTAL, Fair Value	458,890	984,788
TOTAL, Unrealized (Losses)	(9,447)	(31,825)
Asset-backed Securities
Available-for-sale debt securities [Abstract]
Amortized cost	284,701	310,700
Gross unrealized gains	1,286	2,585
Gross unrealized (losses)	(1,598)	(257)
Fair Value	284,389	313,028
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	148,254	146,638
LESS THAN 12 MONTHS, Unrealized (Losses)	(1,539)	(257)
OVER 12 MONTHS, Fair Value	12,184	0
OVER 12 MONTHS, Unrealized (Losses)	(59)	0
TOTAL, Fair Value	160,438	146,638
TOTAL, Unrealized (Losses)	(1,598)	(257)
Mortgage-backed Securities
Available-for-sale debt securities [Abstract]
Amortized cost	419,024	378,301
Gross unrealized gains	1,064	0
Gross unrealized (losses)	(6,595)	(14,236)
Fair Value	413,493	364,065
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	32,722	121,217
LESS THAN 12 MONTHS, Unrealized (Losses)	(13)	(3,292)
OVER 12 MONTHS, Fair Value	282,789	242,849
OVER 12 MONTHS, Unrealized (Losses)	(6,582)	(10,944)
TOTAL, Fair Value	315,511	364,066
TOTAL, Unrealized (Losses)	(6,595)	(14,236)
Debt Securities
Available-for-sale debt securities [Abstract]
Amortized cost	1,508,510	1,892,630
Gross unrealized gains	4,450	3,461
Gross unrealized (losses)	(17,804)	(47,866)
Fair Value	1,495,156	1,848,225
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	233,031	948,681
LESS THAN 12 MONTHS, Unrealized (Losses)	(1,583)	(18,622)
OVER 12 MONTHS, Fair Value	772,919	601,825
OVER 12 MONTHS, Unrealized (Losses)	(16,221)	(29,244)
TOTAL, Fair Value	1,005,950	1,550,506
TOTAL, Unrealized (Losses)	(17,804)	(47,866)
Common Equities and Mutual Funds
Available-for-sale debt securities [Abstract]
Amortized cost	3,592	3,172
Gross unrealized gains	619	635
Gross unrealized (losses)	0	(7)
Fair Value	4,211	3,800
Available-for-sale securities in a continuous unrealized loss position [Abstract]
LESS THAN 12 MONTHS, Fair Value	0	1,818
LESS THAN 12 MONTHS, Unrealized (Losses)	0	(7)
OVER 12 MONTHS, Fair Value	0	0
OVER 12 MONTHS, Unrealized (Losses)	0	0
TOTAL, Fair Value	0	1,818
TOTAL, Unrealized (Losses)	$ 0	$ (7)