Loans - Risk Category of Loans by Class of Loans (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 2,114,932	$ 2,016,141
Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	2,050,907	1,960,095
Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	22,232	15,808
Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	41,793	40,238
Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	41,793	40,238
Commercial Loans [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	900,817	822,616
Commercial Loans [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	846,398	778,206
Commercial Loans [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	21,130	14,223
Commercial Loans [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	33,289	30,187
Commercial Loans [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	33,289	30,187
Commercial Loans [Member] | Multifamily [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	141,004	120,480
Commercial Loans [Member] | Multifamily [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	138,238	118,716
Commercial Loans [Member] | Multifamily [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	2,491	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	396,624	381,611
Commercial Loans [Member] | Nonresidential [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	379,794	367,553
Commercial Loans [Member] | Nonresidential [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	5,162	6,394
Commercial Loans [Member] | Nonresidential [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	11,668	7,664
Commercial Loans [Member] | Nonresidential [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	11,668	7,664
Commercial Loans [Member] | Land [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	16,887	15,162
Commercial Loans [Member] | Land [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	16,887	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	127,691	116,863
Commercial Loans [Member] | Construction [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	119,065	116,460
Commercial Loans [Member] | Construction [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	8,626	403
Commercial Loans [Member] | Secured [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	212,173	177,994
Commercial Loans [Member] | Secured [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	186,069	149,912
Commercial Loans [Member] | Secured [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	4,851	6,092
Commercial Loans [Member] | Secured [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	21,253	21,990
Commercial Loans [Member] | Secured [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	21,253	21,990
Commercial Loans [Member] | Unsecured [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	6,438	10,506
Commercial Loans [Member] | Unsecured [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	6,345	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	929,206	920,031
Residential Mortgage Loans [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	921,402	911,063
Residential Mortgage Loans [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,102	1,585
Residential Mortgage Loans [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	6,702	7,383
Residential Mortgage Loans [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	6,702	7,383
Residential Mortgage Loans [Member] | Construction [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	40,623	49,092
Residential Mortgage Loans [Member] | Construction [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	40,623	49,092
Residential Mortgage Loans [Member] | One-to Four-Family [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	888,583	870,939
Residential Mortgage Loans [Member] | One-to Four-Family [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	880,779	861,971
Residential Mortgage Loans [Member] | One-to Four-Family [Member] | Special Mention [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,102	1,585
Residential Mortgage Loans [Member] | One-to Four-Family [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	6,702	7,383
Residential Mortgage Loans [Member] | One-to Four-Family [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	6,702	7,383
Consumer Loans [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	284,909	273,494
Consumer Loans [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	283,107	270,826
Consumer Loans [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,802	2,668
Consumer Loans [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,802	2,668
Consumer Loans [Member] | Home Equity [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	187,771	195,852
Consumer Loans [Member] | Home Equity [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	186,276	193,733
Consumer Loans [Member] | Home Equity [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,495	2,119
Consumer Loans [Member] | Home Equity [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,495	2,119
Consumer Loans [Member] | Auto [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	83,829	64,364
Consumer Loans [Member] | Auto [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	83,766	64,209
Consumer Loans [Member] | Auto [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	63	155
Consumer Loans [Member] | Auto [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	63	155
Consumer Loans [Member] | Marine [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,404	1,526
Consumer Loans [Member] | Marine [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1,223	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	4,879	5,696
Consumer Loans [Member] | Recreational Vehicle [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	4,817	5,488
Consumer Loans [Member] | Recreational Vehicle [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	62	208
Consumer Loans [Member] | Recreational Vehicle [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	62	208
Consumer Loans [Member] | Other [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	7,026	6,056
Consumer Loans [Member] | Other [Member] | Unclassified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	7,025	6,051
Consumer Loans [Member] | Other [Member] | Substandard [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	1	5
Consumer Loans [Member] | Other [Member] | Classified [Member]
Financing Receivable Recorded Investment [Line Items]
Total loans	$ 1	$ 5