Securities Available for Sale - Schedule of Amortized Cost and Fair Value of Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	$ 1,912,682	$ 1,765,284
Gross Unrealized Gains	60,211	22,421
Gross Unrealized Losses	(267)	(4,497)
Estimated Fair Value	1,972,626	1,783,208
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	49,795	94,178
Gross Unrealized Gains	553	599
Gross Unrealized Losses	0	(32)
Estimated Fair Value	50,348	94,745
Mortgage-backed securities \| Government National Mortgage Association
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	546,739	501,139
Gross Unrealized Gains	13,251	3,374
Gross Unrealized Losses	(101)	(3,027)
Estimated Fair Value	559,889	501,486
Mortgage-backed securities \| Federal Home Loan Mortgage Corporation
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	575,530	463,974
Gross Unrealized Gains	19,400	8,840
Gross Unrealized Losses	(2)	(770)
Estimated Fair Value	594,928	472,044
Mortgage-backed securities \| Federal National Mortgage Association
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	385,813	322,340
Gross Unrealized Gains	15,840	5,409
Gross Unrealized Losses	(6)	(398)
Estimated Fair Value	401,647	327,351
Mortgage-backed securities \| Small Business Assistance Program
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	295,662	316,502
Gross Unrealized Gains	9,802	3,674
Gross Unrealized Losses	(158)	(154)
Estimated Fair Value	305,306	320,022
States and political subdivision securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	58,137	66,145
Gross Unrealized Gains	1,319	494
Gross Unrealized Losses	0	(116)
Estimated Fair Value	59,456	66,523
Other
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	1,006	1,006
Gross Unrealized Gains	46	31
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 1,052	$ 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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