Loans Receivable - Additional Information (Detail)	3 Months Ended								12 Months Ended
	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2017 USD ($) Contract	Sep. 30, 2016 USD ($) Contract	Sep. 30, 2015 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reclassifications of nonaccretable discounts									$ 215,000	$ 163,000
Allowance for loan losses recorded for acquired loans									155,000	236,000
Mortgage loans serviced for others									61,167,000	78,497,000
Purchased impaired loans and non accrual loans	$ 14,263,000				$ 19,315,000				14,263,000	19,315,000	$ 20,105,000
Additional interest income under loan agreements									$ 449,000	782,000	188,000
Criteria in internal rating system									Ten-point
Categories considered as not criticized									six
Days past due over which loans are considered as substandard									90 days
Minimum internal review amount	750,000								$ 750,000
Minimum external review amount	1,000,000								1,000,000
Charge-offs									3,923,000	2,795,000	2,083,000
Increase in outstanding loan balances									25,765,000	973,000	49,790,000
Provision for loan losses	1,100,000	$ 750,000	$ 750,000	$ 750,000	$ 750,000	$ 600,000	$ 600,000	$ 600,000	$ 3,350,000	$ 2,550,000	2,075,000
Number of Contracts | Contract									11	17
Number of troubled debt restructuring loans granted terms and rate concessions | Contract									6	8
Troubled debt restructurings granted terms and rate concession									$ 832,000,000,000	$ 2,400,000
Number of troubled debt restructuring loans granted terms concessions | Contract									3	7
Troubled debt restructurings granted terms concession									$ 1,200,000	$ 1,500,000
Number of troubled debt restructuring loans granted interest rate concession | Contract									2	2
Troubled debt restructurings loans granted interest rate concession									$ 368,000	$ 29,000
Number of troubled debt restructurings, loan modified, defaulted within one year of modification | Contract									0	0
Consumer Residential Mortgages [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Foreclosed assets	819,000,000,000								$ 819,000,000,000
Formal foreclosure proceeding assets	$ 2,200,000								2,200,000
Auto Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs									2,009,000	$ 1,262,000	596,000
Increase in outstanding loan balances									29,900,000	30,900,000	61,600,000
Provision for loan losses									$ 1,150,000	1,326,000	1,620,000
Number of Contracts | Contract									2
Commercial Real Estate Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Charge-offs									$ 1,352,000	266,000	65,000
Provision for loan losses									$ 2,231,000	$ 395,000	$ (11,000)
Maximum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of unallocated allowance	10.00%								10.00%