Securities (Amortized Cost, Estimated Fair Value, and Gross Unrealized Gains and Losses of AFS and HTM Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Marketable Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 1,529,605	$ 1,191,455
Available-for-sale Securities, Gross Unrealized Gains	31,717	15,927
Available-for-sale Securities, Gross Unrealized Losses	372	2,519
Available-for-sale Securities, Estimated Fair Value	1,560,950	1,204,863
MBS [Member]
Marketable Securities [Line Items]
Available-for-sale Securities, Amortized Cost	1,149,922	923,256
Available-for-sale Securities, Gross Unrealized Gains	31,212	15,571
Available-for-sale Securities, Gross Unrealized Losses	331	2,340
Available-for-sale Securities, Estimated Fair Value	1,180,803	936,487
GSE Debentures [Member]
Marketable Securities [Line Items]
Available-for-sale Securities, Amortized Cost	369,967	249,828
Available-for-sale Securities, Gross Unrealized Gains	414	304
Available-for-sale Securities, Gross Unrealized Losses	41	178
Available-for-sale Securities, Estimated Fair Value	370,340	249,954
Municipal Bonds [Member]
Marketable Securities [Line Items]
Available-for-sale Securities, Amortized Cost	9,716	18,371
Available-for-sale Securities, Gross Unrealized Gains	91	52
Available-for-sale Securities, Gross Unrealized Losses	0	1
Available-for-sale Securities, Estimated Fair Value	$ 9,807	$ 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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