Loans Receivable And Allowance For Credit Losses (Recorded Investment In Classified Loans) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	$ 7,508,001	$ 7,425,973
Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	66,654	69,780
Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	31,151	29,956
One- to Four-Family Segment [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	6,613,698	6,530,681
One- to Four-Family Segment [Member] | Originated [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	3,966,385	3,895,783
One- to Four-Family Segment [Member] | Originated [Member] | Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	10,586	12,941
One- to Four-Family Segment [Member] | Originated [Member] | Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	17,308	15,628
One- to Four-Family Segment [Member] | Correspondent Purchased [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	2,417,555	2,381,414
One- to Four-Family Segment [Member] | Correspondent Purchased [Member] | Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	3,092	2,349
One- to Four-Family Segment [Member] | Correspondent Purchased [Member] | Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	3,741	2,785
One- to Four-Family Segment [Member] | Bulk Purchased [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	229,758	253,484
One- to Four-Family Segment [Member] | Bulk Purchased [Member] | Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	0	102
One- to Four-Family Segment [Member] | Bulk Purchased [Member] | Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	4,910	5,294
Commercial Segment [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	769,031	763,722
Commercial Segment [Member] | Commercial Real Estate [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	707,164	703,139
Commercial Segment [Member] | Commercial Real Estate [Member] | Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	51,416	52,891
Commercial Segment [Member] | Commercial Real Estate [Member] | Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	2,232	2,472
Commercial Segment [Member] | Commercial and Industrial [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	61,867	60,583
Commercial Segment [Member] | Commercial and Industrial [Member] | Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	1,082	1,215
Commercial Segment [Member] | Commercial and Industrial [Member] | Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	2,301	3,057
Consumer Segment [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	125,272	131,570
Consumer Segment [Member] | Home Equity [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	114,475	120,436
Consumer Segment [Member] | Home Equity [Member] | Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	471	280
Consumer Segment [Member] | Home Equity [Member] | Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	637	696
Consumer Segment [Member] | Other [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	10,797	11,134
Consumer Segment [Member] | Other [Member] | Special Mention [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	7	2
Consumer Segment [Member] | Other [Member] | Substandard [Member]
Financing Receivable, Credit Quality Indicator [Line Items]
Financing receivable, total recorded investment	$ 22	$ 24