Securities (Amortized Cost, Estimated Fair Value, and Gross Unrealized Gains and Losses of AFS and HTM Securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Sep. 30, 2019
Marketable Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 1,186,225	$ 1,191,455
Available-for-sale Securities, Gross Unrealized Gains	33,894	15,927
Available-for-sale Securities, Gross Unrealized Losses	65	2,519
Available-for-sale Securities, Estimated Fair Value	1,220,054	1,204,863
MBS [Member]
Marketable Securities [Line Items]
Available-for-sale Securities, Amortized Cost	949,348	923,256
Available-for-sale Securities, Gross Unrealized Gains	33,304	15,571
Available-for-sale Securities, Gross Unrealized Losses	65	2,340
Available-for-sale Securities, Estimated Fair Value	982,587	936,487
GSE Debentures [Member]
Marketable Securities [Line Items]
Available-for-sale Securities, Amortized Cost	225,020	249,828
Available-for-sale Securities, Gross Unrealized Gains	480	304
Available-for-sale Securities, Gross Unrealized Losses	0	178
Available-for-sale Securities, Estimated Fair Value	225,500	249,954
Municipal Bonds [Member]
Marketable Securities [Line Items]
Available-for-sale Securities, Amortized Cost	11,857	18,371
Available-for-sale Securities, Gross Unrealized Gains	110	52
Available-for-sale Securities, Gross Unrealized Losses	0	1
Available-for-sale Securities, Estimated Fair Value	$ 11,967	$ 18,422

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).

+ References

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