Securities Available for Sale - Schedule of Amortized Cost and Fair Value of Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	$ 1,941,922	$ 1,765,284
Gross Unrealized Gains	49,665	22,421
Gross Unrealized Losses	(1,560)	(4,497)
Estimated Fair Value	1,990,027	1,783,208
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	69,629	94,178
Gross Unrealized Gains	954	599
Gross Unrealized Losses	0	(32)
Estimated Fair Value	70,583	94,745
Mortgage-backed Securities \| Government National Mortgage Association
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	566,747	501,139
Gross Unrealized Gains	11,340	3,374
Gross Unrealized Losses	(733)	(3,027)
Estimated Fair Value	577,354	501,486
Mortgage-backed Securities \| Federal Home Loan Mortgage Corporation
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	556,535	463,974
Gross Unrealized Gains	15,747	8,840
Gross Unrealized Losses	(731)	(770)
Estimated Fair Value	571,551	472,044
Mortgage-backed Securities \| Federal National Mortgage Association
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	386,996	322,340
Gross Unrealized Gains	15,337	5,409
Gross Unrealized Losses	(27)	(398)
Estimated Fair Value	402,306	327,351
Mortgage-backed Securities \| Small Business Assistance Program
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	300,498	316,502
Gross Unrealized Gains	5,725	3,674
Gross Unrealized Losses	(42)	(154)
Estimated Fair Value	306,181	320,022
States and political subdivision securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	60,511	66,145
Gross Unrealized Gains	517	494
Gross Unrealized Losses	(27)	(116)
Estimated Fair Value	61,001	66,523
Other
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	1,006	1,006
Gross Unrealized Gains	45	31
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 1,051	$ 1,037

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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