Loans - Risk Category of Loans by Class of Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 2,163,897	$ 2,016,141
Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	2,116,569	1,960,095
Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	6,968	15,808
Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	40,360	40,238
Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	40,360	40,238
Commercial Loans [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	931,767	822,616
Commercial Loans [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	893,028	778,206
Commercial Loans [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	6,372	14,223
Commercial Loans [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	32,367	30,187
Commercial Loans [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	32,367	30,187
Commercial Loans [Member] | Multifamily [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	139,938	120,480
Commercial Loans [Member] | Multifamily [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	138,378	118,716
Commercial Loans [Member] | Multifamily [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,285	1,334
Commercial Loans [Member] | Multifamily [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	275	430
Commercial Loans [Member] | Multifamily [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	275	430
Commercial Loans [Member] | Nonresidential [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	408,938	381,611
Commercial Loans [Member] | Nonresidential [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	393,138	367,553
Commercial Loans [Member] | Nonresidential [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	3,225	6,394
Commercial Loans [Member] | Nonresidential [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	12,575	7,664
Commercial Loans [Member] | Nonresidential [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	12,575	7,664
Commercial Loans [Member] | Land [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	16,129	15,162
Commercial Loans [Member] | Land [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	16,129	15,153
Commercial Loans [Member] | Land [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans		9
Commercial Loans [Member] | Land [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans		9
Commercial Loans [Member] | Construction [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	132,961	116,863
Commercial Loans [Member] | Construction [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	132,961	116,460
Commercial Loans [Member] | Construction [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans		403
Commercial Loans [Member] | Secured [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	226,657	177,994
Commercial Loans [Member] | Secured [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	205,371	149,912
Commercial Loans [Member] | Secured [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,862	6,092
Commercial Loans [Member] | Secured [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	19,424	21,990
Commercial Loans [Member] | Secured [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	19,424	21,990
Commercial Loans [Member] | Unsecured [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	7,144	10,506
Commercial Loans [Member] | Unsecured [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	7,051	10,412
Commercial Loans [Member] | Unsecured [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	93	94
Commercial Loans [Member] | Unsecured [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	93	94
Residential Mortgage Loans [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	949,022	920,031
Residential Mortgage Loans [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	942,205	911,063
Residential Mortgage Loans [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	596	1,585
Residential Mortgage Loans [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	6,221	7,383
Residential Mortgage Loans [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	6,221	7,383
Residential Mortgage Loans [Member] | Construction [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	39,396	49,092
Residential Mortgage Loans [Member] | Construction [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	39,396	49,092
Residential Mortgage Loans [Member] | One-to Four-Family [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	909,626	870,939
Residential Mortgage Loans [Member] | One-to Four-Family [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	902,809	861,971
Residential Mortgage Loans [Member] | One-to Four-Family [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	596	1,585
Residential Mortgage Loans [Member] | One-to Four-Family [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	6,221	7,383
Residential Mortgage Loans [Member] | One-to Four-Family [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	6,221	7,383
Consumer Loans [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	283,108	273,494
Consumer Loans [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	281,336	270,826
Consumer Loans [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,772	2,668
Consumer Loans [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,772	2,668
Consumer Loans [Member] | Home Equity [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	187,878	195,852
Consumer Loans [Member] | Home Equity [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	186,448	193,733
Consumer Loans [Member] | Home Equity [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,430	2,119
Consumer Loans [Member] | Home Equity [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,430	2,119
Consumer Loans [Member] | Auto [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	82,219	64,364
Consumer Loans [Member] | Auto [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	82,177	64,209
Consumer Loans [Member] | Auto [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	42	155
Consumer Loans [Member] | Auto [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	42	155
Consumer Loans [Member] | Marine [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,276	1,526
Consumer Loans [Member] | Marine [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,095	1,345
Consumer Loans [Member] | Marine [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	181	181
Consumer Loans [Member] | Marine [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	181	181
Consumer Loans [Member] | Recreational Vehicle [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	4,546	5,696
Consumer Loans [Member] | Recreational Vehicle [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	4,448	5,488
Consumer Loans [Member] | Recreational Vehicle [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	98	208
Consumer Loans [Member] | Recreational Vehicle [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	98	208
Consumer Loans [Member] | Other [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	7,189	6,056
Consumer Loans [Member] | Other [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	7,168	6,051
Consumer Loans [Member] | Other [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	21	5
Consumer Loans [Member] | Other [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 21	$ 5