Securities (Amortized Cost, Estimated Fair Value, and Gross Unrealized Gains and Losses of AFS and HTM Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 1,209,516	$ 1,191,455
Available-for-sale Securities, Gross Unrealized Gains	26,975	15,927
Available-for-sale Securities, Gross Unrealized Losses	454	2,519
Available-for-sale Securities, Estimated Fair Value	1,236,037	1,204,863
MBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	947,549	923,256
Available-for-sale Securities, Gross Unrealized Gains	26,223	15,571
Available-for-sale Securities, Gross Unrealized Losses	454	2,340
Available-for-sale Securities, Estimated Fair Value	973,318	936,487
GSE Debentures [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	250,080	249,828
Available-for-sale Securities, Gross Unrealized Gains	699	304
Available-for-sale Securities, Gross Unrealized Losses	0	178
Available-for-sale Securities, Estimated Fair Value	250,779	249,954
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	11,887	18,371
Available-for-sale Securities, Gross Unrealized Gains	53	52
Available-for-sale Securities, Gross Unrealized Losses	0	1
Available-for-sale Securities, Estimated Fair Value	$ 11,940	$ 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).

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