Fair Value Measurements and Disclosures about Fair Value of Financial Instruments (Balances Of Financial Assets Measured At Fair Value On Recurring And Nonrecurring) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	$ 0		$ 7,175
Total securities available for sale	208,071		178,099
Loans servicing rights	2,445	$ 2,116	1,389	$ 1,026	$ 783	$ 310
Other real estate owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total other real estate owned	64
Residential real estate | Other real estate owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total other real estate owned	64
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities			7,175
Total securities available for sale	208,071		178,099
Fair Value, Measurements, Recurring | Agent Bonds And Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	9,869
Fair Value, Measurements, Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	44,180		36,736
Fair Value, Measurements, Recurring | Agency CMO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	11,081		14,576
Fair Value, Measurements, Recurring | Privately issued CMO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	1,576		2,001
Fair Value, Measurements, Recurring | Privately-issued ABS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	2,217		3,448
Fair Value, Measurements, Recurring | SBA Certificates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	1,408		912
Fair Value, Measurements, Recurring | Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	137,740		120,426
Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	12,457		10,841
Total loans held for sale	25,276		25,635
Loans servicing rights	2,445		1,389
Total other real estate owned			852
Fair Value, Measurements, Nonrecurring | Residential real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	5,018		4,967
Fair Value, Measurements, Nonrecurring | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	6,801		5,477
Total other real estate owned			260
Fair Value, Measurements, Nonrecurring | Commercial business
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	343		192
Fair Value, Measurements, Nonrecurring | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	268		175
Fair Value, Measurements, Nonrecurring | Land and Land Development [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	27		30
Total other real estate owned			282
Fair Value, Measurements, Nonrecurring | Small Business Administration Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total loans held for sale	22,274		24,908
Fair Value, Measurements, Nonrecurring | Residential mortgage loans held for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total loans held for sale	3,002		727
Total other real estate owned			310
Level 1 | Other real estate owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total other real estate owned	0
Level 1 | Residential real estate | Other real estate owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total other real estate owned	0
Level 1 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities			0
Total securities available for sale	0		0
Level 1 | Fair Value, Measurements, Recurring | Agent Bonds And Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	0
Level 1 | Fair Value, Measurements, Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	0		0
Level 1 | Fair Value, Measurements, Recurring | Agency CMO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	0		0
Level 1 | Fair Value, Measurements, Recurring | Privately issued CMO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	0		0
Level 1 | Fair Value, Measurements, Recurring | Privately-issued ABS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	0		0
Level 1 | Fair Value, Measurements, Recurring | SBA Certificates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	0		0
Level 1 | Fair Value, Measurements, Recurring | Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	0		0
Level 1 | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	0		0
Total loans held for sale	0		0
Loans servicing rights	0		0
Total other real estate owned			0
Level 1 | Fair Value, Measurements, Nonrecurring | Residential real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	0		0
Level 1 | Fair Value, Measurements, Nonrecurring | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	0		0
Total other real estate owned			0
Level 1 | Fair Value, Measurements, Nonrecurring | Commercial business
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	0		0
Level 1 | Fair Value, Measurements, Nonrecurring | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	0		0
Level 1 | Fair Value, Measurements, Nonrecurring | Land and Land Development [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	0		0
Total other real estate owned			0
Level 1 | Fair Value, Measurements, Nonrecurring | Small Business Administration Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total loans held for sale	0		0
Level 1 | Fair Value, Measurements, Nonrecurring | Residential mortgage loans held for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total loans held for sale	0		0
Total other real estate owned			0
Level 2 | Other real estate owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total other real estate owned	0
Level 2 | Residential real estate | Other real estate owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total other real estate owned	0
Level 2 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities			7,175
Total securities available for sale	208,071		178,099
Level 2 | Fair Value, Measurements, Recurring | Agent Bonds And Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	9,869
Level 2 | Fair Value, Measurements, Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	44,180		36,736
Level 2 | Fair Value, Measurements, Recurring | Agency CMO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	11,081		14,576
Level 2 | Fair Value, Measurements, Recurring | Privately issued CMO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	1,576		2,001
Level 2 | Fair Value, Measurements, Recurring | Privately-issued ABS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	2,217		3,448
Level 2 | Fair Value, Measurements, Recurring | SBA Certificates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	1,408		912
Level 2 | Fair Value, Measurements, Recurring | Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	137,740		120,426
Level 2 | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	0		0
Total loans held for sale	3,002		25,635
Loans servicing rights	0		0
Total other real estate owned			0
Level 2 | Fair Value, Measurements, Nonrecurring | Residential real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	0		0
Level 2 | Fair Value, Measurements, Nonrecurring | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	0		0
Total other real estate owned			0
Level 2 | Fair Value, Measurements, Nonrecurring | Commercial business
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	0		0
Level 2 | Fair Value, Measurements, Nonrecurring | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	0		0
Level 2 | Fair Value, Measurements, Nonrecurring | Land and Land Development [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	0		0
Total other real estate owned			0
Level 2 | Fair Value, Measurements, Nonrecurring | Small Business Administration Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total loans held for sale	0		24,908
Level 2 | Fair Value, Measurements, Nonrecurring | Residential mortgage loans held for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total loans held for sale	3,002		727
Total other real estate owned			0
Level 3 | Other real estate owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total other real estate owned	64
Level 3 | Residential real estate | Other real estate owned
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total other real estate owned	64
Level 3 | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities			0
Total securities available for sale	0		0
Level 3 | Fair Value, Measurements, Recurring | Agent Bonds And Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	0
Level 3 | Fair Value, Measurements, Recurring | Agency mortgage-backed
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	0		0
Level 3 | Fair Value, Measurements, Recurring | Agency CMO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	0		0
Level 3 | Fair Value, Measurements, Recurring | Privately issued CMO
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	0		0
Level 3 | Fair Value, Measurements, Recurring | Privately-issued ABS
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	0		0
Level 3 | Fair Value, Measurements, Recurring | SBA Certificates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	0		0
Level 3 | Fair Value, Measurements, Recurring | Municipal obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total securities available for sale	0		0
Level 3 | Fair Value, Measurements, Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	12,457		10,841
Total loans held for sale	22,274		0
Loans servicing rights	2,445		1,389
Total other real estate owned			852
Level 3 | Fair Value, Measurements, Nonrecurring | Residential real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	5,018		4,967
Level 3 | Fair Value, Measurements, Nonrecurring | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	6,801		5,477
Total other real estate owned			260
Level 3 | Fair Value, Measurements, Nonrecurring | Commercial business
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	343		192
Level 3 | Fair Value, Measurements, Nonrecurring | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	268		175
Level 3 | Fair Value, Measurements, Nonrecurring | Land and Land Development [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total impaired loans	27		30
Total other real estate owned			282
Level 3 | Fair Value, Measurements, Nonrecurring | Small Business Administration Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total loans held for sale	22,274		0
Level 3 | Fair Value, Measurements, Nonrecurring | Residential mortgage loans held for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total loans held for sale	$ 0		0
Total other real estate owned			$ 310