Securities Available for Sale - Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	$ 1,726,962	$ 1,765,284
Gross Unrealized Gains	47,790	22,421
Gross Unrealized Losses	(126)	(4,497)
Estimated Fair Value	1,774,626	1,783,208
U.S. Treasury securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	49,924	94,178
Gross Unrealized Gains	228	599
Gross Unrealized Losses	0	(32)
Estimated Fair Value	50,152	94,745
U.S. Agency securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	24,974
Gross Unrealized Gains	86
Gross Unrealized Losses	0
Estimated Fair Value	25,060
Mortgage backed-securities \| Government National Mortgage Association
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	485,689	501,139
Gross Unrealized Gains	11,481	3,374
Gross Unrealized Losses	(43)	(3,027)
Estimated Fair Value	497,127	501,486
Mortgage backed-securities \| Federal Home Loan Mortgage Corporation
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	578,650	463,974
Gross Unrealized Gains	18,919	8,840
Gross Unrealized Losses	(9)	(770)
Estimated Fair Value	597,560	472,044
Mortgage backed-securities \| Federal National Mortgage Association
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	287,842	322,340
Gross Unrealized Gains	7,788	5,409
Gross Unrealized Losses	(16)	(398)
Estimated Fair Value	295,614	327,351
Mortgage backed-securities \| Small Business Assistance Program
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	244,653	316,502
Gross Unrealized Gains	7,884	3,674
Gross Unrealized Losses	(58)	(154)
Estimated Fair Value	252,479	320,022
States and political subdivision securities
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	54,224	66,145
Gross Unrealized Gains	1,356	494
Gross Unrealized Losses	0	(116)
Estimated Fair Value	55,580	66,523
Other
Debt Securities, Available-for-sale, Fair Value to Amortized Cost [Abstract]
Amortized Cost	1,006	1,006
Gross Unrealized Gains	48	31
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 1,054	$ 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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