Loans Receivable And Allowance For Credit Losses (Recorded Investment in Loans, Past Due) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	$ 24,148	$ 21,193
Financing receivable, current loans	7,414,494	7,404,780
Financing receivable, total recorded investment	7,438,642	7,425,973
Financial Asset, 30 to 89 Days Past Due [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	18,185	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	5,963	6,264
One- to Four-Family Segment [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total recorded investment	6,564,726	6,530,681
One- to Four-Family Segment [Member] | Originated [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	12,507	10,448
Financing receivable, current loans	3,938,497	3,885,335
Financing receivable, total recorded investment	3,951,004	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	8,980	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	3,527	3,261
One- to Four-Family Segment [Member] | Correspondent Purchased [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	5,574	3,785
Financing receivable, current loans	2,369,379	2,377,629
Financing receivable, total recorded investment	2,374,953	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	4,179	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	1,395	1,023
One- to Four-Family Segment [Member] | Bulk Purchased [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	4,046	5,108
Financing receivable, current loans	234,723	248,376
Financing receivable, total recorded investment	238,769	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	3,345	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	701	1,484
Commercial Segment [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total recorded investment	744,674	763,722
Commercial Segment [Member] | Commercial Real Estate [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	963	762
Financing receivable, current loans	687,095	702,377
Financing receivable, total recorded investment	688,058	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	963	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	0	0
Commercial Segment [Member] | Commercial and Industrial [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	231	243
Financing receivable, current loans	56,385	60,340
Financing receivable, total recorded investment	56,616	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	231	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	0	173
Consumer Segment [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total recorded investment	129,242	131,570
Consumer Segment [Member] | Home Equity [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	770	748
Financing receivable, current loans	117,637	119,688
Financing receivable, total recorded investment	118,407	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	442	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	328	302
Consumer Segment [Member] | Other [Member]
Financing Receivable, Past Due [Line Items]
Financing receivable, total delinquent loans	57	99
Financing receivable, current loans	10,778	11,035
Financing receivable, total recorded investment	10,835	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	45	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	$ 12	$ 21