Securities (Amortized Cost, Estimated Fair Value, and Gross Unrealized Gains and Losses of AFS and HTM Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	$ 1,216,776	$ 1,191,455
Available-for-sale Securities, Gross Unrealized Gains	15,305	15,927
Available-for-sale Securities, Gross Unrealized Losses	2,494	2,519
Available-for-sale Securities, Estimated Fair Value	1,229,587	1,204,863
MBS [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	924,732	923,256
Available-for-sale Securities, Gross Unrealized Gains	14,983	15,571
Available-for-sale Securities, Gross Unrealized Losses	2,398	2,340
Available-for-sale Securities, Estimated Fair Value	937,317	936,487
GSE Debentures [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	274,994	249,828
Available-for-sale Securities, Gross Unrealized Gains	257	304
Available-for-sale Securities, Gross Unrealized Losses	96	178
Available-for-sale Securities, Estimated Fair Value	275,155	249,954
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost	17,050	18,371
Available-for-sale Securities, Gross Unrealized Gains	65	52
Available-for-sale Securities, Gross Unrealized Losses	0	1
Available-for-sale Securities, Estimated Fair Value	$ 17,115	$ 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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