Loans - Presentation of Age Analysis of Past-Due Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	$ 14,881	$ 16,657
Current Loans	2,149,016	1,999,484
Total loans	2,163,897	2,016,141
30-59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	4,554	6,287
60-89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	2,127	1,750
Greater than 90 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	8,200	8,620
Commercial Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	2,946	1,731
Current Loans	928,821	820,885
Total loans	931,767	822,616
Commercial Loans [Member] | 30-59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	95	134
Commercial Loans [Member] | 60-89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due		4
Commercial Loans [Member] | Greater than 90 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	2,851	1,593
Commercial Loans [Member] | Multifamily [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	275	275
Current Loans	139,663	120,205
Total loans	139,938	120,480
Commercial Loans [Member] | Multifamily [Member] | Greater than 90 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	275	275
Commercial Loans [Member] | Nonresidential [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	1,087	1,219
Current Loans	407,851	380,392
Total loans	408,938	381,611
Commercial Loans [Member] | Nonresidential [Member] | 30-59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due		20
Commercial Loans [Member] | Nonresidential [Member] | Greater than 90 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	1,087	1,199
Commercial Loans [Member] | Land [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due		9
Current Loans	16,129	15,153
Total loans	16,129	15,162
Commercial Loans [Member] | Land [Member] | Greater than 90 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due		9
Commercial Loans [Member] | Construction [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Current Loans	132,961	116,863
Total loans	132,961	116,863
Commercial Loans [Member] | Secured [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	1,584	228
Current Loans	225,073	177,766
Total loans	226,657	177,994
Commercial Loans [Member] | Secured [Member] | 30-59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	95	114
Commercial Loans [Member] | Secured [Member] | 60-89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due		4
Commercial Loans [Member] | Secured [Member] | Greater than 90 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	1,489	110
Commercial Loans [Member] | Unsecured [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Current Loans	7,144	10,506
Total loans	7,144	10,506
Residential Mortgage Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	8,900	11,031
Current Loans	940,122	909,000
Total loans	949,022	920,031
Residential Mortgage Loans [Member] | 30-59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	3,478	4,704
Residential Mortgage Loans [Member] | 60-89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	1,757	1,523
Residential Mortgage Loans [Member] | Greater than 90 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	3,665	4,804
Residential Mortgage Loans [Member] | Construction [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Current Loans	39,396	49,092
Total loans	39,396	49,092
Residential Mortgage Loans [Member] | One-to Four-Family [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	8,900	11,031
Current Loans	900,726	859,908
Total loans	909,626	870,939
Residential Mortgage Loans [Member] | One-to Four-Family [Member] | 30-59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	3,478	4,704
Residential Mortgage Loans [Member] | One-to Four-Family [Member] | 60-89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	1,757	1,523
Residential Mortgage Loans [Member] | One-to Four-Family [Member] | Greater than 90 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	3,665	4,804
Consumer Loans [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	3,035	3,895
Current Loans	280,073	269,599
Total loans	283,108	273,494
Consumer Loans [Member] | 30-59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	981	1,449
Consumer Loans [Member] | 60-89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	370	223
Consumer Loans [Member] | Greater than 90 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	1,684	2,223
Consumer Loans [Member] | Home Equity [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	2,192	3,097
Current Loans	185,686	192,755
Total loans	187,878	195,852
Consumer Loans [Member] | Home Equity [Member] | 30-59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	516	1,184
Consumer Loans [Member] | Home Equity [Member] | 60-89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	324	120
Consumer Loans [Member] | Home Equity [Member] | Greater than 90 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	1,352	1,793
Consumer Loans [Member] | Automobile [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	411	369
Current Loans	81,808	63,995
Total loans	82,219	64,364
Consumer Loans [Member] | Automobile [Member] | 30-59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	334	187
Consumer Loans [Member] | Automobile [Member] | 60-89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	35	100
Consumer Loans [Member] | Automobile [Member] | Greater than 90 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	42	82
Consumer Loans [Member] | Marine [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	181	181
Current Loans	1,095	1,345
Total loans	1,276	1,526
Consumer Loans [Member] | Marine [Member] | Greater than 90 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	181	181
Consumer Loans [Member] | Recreational Vehicle [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	228	212
Current Loans	4,318	5,484
Total loans	4,546	5,696
Consumer Loans [Member] | Recreational Vehicle [Member] | 30-59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	129	47
Consumer Loans [Member] | Recreational Vehicle [Member] | 60-89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	8
Consumer Loans [Member] | Recreational Vehicle [Member] | Greater than 90 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	91	165
Consumer Loans [Member] | Other [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	23	36
Current Loans	7,166	6,020
Total loans	7,189	6,056
Consumer Loans [Member] | Other [Member] | 30-59 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	2	31
Consumer Loans [Member] | Other [Member] | 60-89 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	3	3
Consumer Loans [Member] | Other [Member] | Greater than 90 Days Past Due [Member]
Financing Receivable Recorded Investment Past Due [Line Items]
Total Past Due	$ 18	$ 2