Loans Receivable And Allowance For Credit Losses (Recorded Investment in Loans, Past Due) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Sep. 30, 2019
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	$ 25,623	$ 21,193
Financing receivable, current loans	7,482,378	7,404,780
Financing receivable, total recorded investment	7,508,001	7,425,973
Financial Asset, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	18,073	14,929
Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	7,550	6,264
One- to Four-Family Segment [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total recorded investment	6,613,698	6,530,681
One- to Four-Family Segment [Member] | Originated [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	12,836	10,448
Financing receivable, current loans	3,953,549	3,885,335
Financing receivable, total recorded investment	3,966,385	3,895,783
One- to Four-Family Segment [Member] | Originated [Member] | Financial Asset, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	8,333	7,187
One- to Four-Family Segment [Member] | Originated [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	4,503	3,261
One- to Four-Family Segment [Member] | Correspondent Purchased [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	5,964	3,785
Financing receivable, current loans	2,411,591	2,377,629
Financing receivable, total recorded investment	2,417,555	2,381,414
One- to Four-Family Segment [Member] | Correspondent Purchased [Member] | Financial Asset, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	4,602	2,762
One- to Four-Family Segment [Member] | Correspondent Purchased [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	1,362	1,023
One- to Four-Family Segment [Member] | Bulk Purchased [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	3,591	5,108
Financing receivable, current loans	226,167	248,376
Financing receivable, total recorded investment	229,758	253,484
One- to Four-Family Segment [Member] | Bulk Purchased [Member] | Financial Asset, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	2,950	3,624
One- to Four-Family Segment [Member] | Bulk Purchased [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	641	1,484
Commercial Segment [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total recorded investment	769,031	763,722
Commercial Segment [Member] | Commercial Real Estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	2,106	762
Financing receivable, current loans	705,058	702,377
Financing receivable, total recorded investment	707,164	703,139
Commercial Segment [Member] | Commercial Real Estate [Member] | Financial Asset, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	1,560	762
Commercial Segment [Member] | Commercial Real Estate [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	546	0
Commercial Segment [Member] | Commercial and Industrial [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	172	243
Financing receivable, current loans	61,695	60,340
Financing receivable, total recorded investment	61,867	60,583
Commercial Segment [Member] | Commercial and Industrial [Member] | Financial Asset, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	0	70
Commercial Segment [Member] | Commercial and Industrial [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	172	173
Consumer Segment [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total recorded investment	125,272	131,570
Consumer Segment [Member] | Home Equity [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	898	748
Financing receivable, current loans	113,577	119,688
Financing receivable, total recorded investment	114,475	120,436
Consumer Segment [Member] | Home Equity [Member] | Financial Asset, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	594	446
Consumer Segment [Member] | Home Equity [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	304	302
Consumer Segment [Member] | Other [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	56	99
Financing receivable, current loans	10,741	11,035
Financing receivable, total recorded investment	10,797	11,134
Consumer Segment [Member] | Other [Member] | Financial Asset, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	34	78
Consumer Segment [Member] | Other [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	$ 22	$ 21