Allowance for Loan Losses - Summary of Activity in Allowance for Loan Losses, Net of Unearned Fees for Periods Indicated (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation	$ 30,007		$ 30,266	$ 28,865
Individually evaluated for impairment	1,524		2,194	2,628
Total allowance for loan losses	31,537	$ 32,868	32,582	31,603	$ 29,524	$ 30,323
Loans pooled for evaluation	4,149,558		3,979,744	3,986,144
Individually evaluated for impairment	22,485		28,047	27,494
Total loans, net of unamortized deferred loan fees and discounts	4,182,348		4,022,014	4,027,436
Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation	14,230		15,357	14,865
Individually evaluated for impairment	117		147	325
Total allowance for loan losses	14,347	14,675	15,620	15,279	13,598	13,758
Loans pooled for evaluation	3,226,848		3,116,086	3,104,594
Individually evaluated for impairment	13,762		16,884	18,025
Total loans, net of unamortized deferred loan fees and discounts	3,247,156		3,143,100	3,132,202
Mortgage Loans on Real Estate | Commercial loans
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation	11,915		12,737	12,552
Individually evaluated for impairment	26		91	268
Total allowance for loan losses	11,941	12,099	12,944	12,879	11,607	11,441
Loans pooled for evaluation	2,727,228		2,606,819	2,586,659
Individually evaluated for impairment	9,005		12,563	13,244
Total loans, net of unamortized deferred loan fees and discounts	2,741,362		2,627,838	2,607,788
Consumer
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation	8,395		8,103	7,061
Individually evaluated for impairment	148		266	446
Total allowance for loan losses	8,549	8,552	8,375	7,514	7,137	8,227
Loans pooled for evaluation	437,068		411,389	414,660
Individually evaluated for impairment	4,222		5,945	4,837
Total loans, net of unamortized deferred loan fees and discounts	442,539		418,982	421,285
Consumer | Home equity lines of credit
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation	5,644		5,838	5,046
Individually evaluated for impairment	84		198	168
Total allowance for loan losses	5,734	5,859	6,042	5,221	5,048	5,800
Loans pooled for evaluation	329,129		322,764	327,649
Individually evaluated for impairment	1,917		2,646	2,188
Total loans, net of unamortized deferred loan fees and discounts	331,872		326,577	331,136
Commercial
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation	4,428		4,309	4,353
Individually evaluated for impairment	1,259		1,781	1,857
Total allowance for loan losses	5,687	6,745	6,090	6,224	6,378	6,512
Loans pooled for evaluation	271,447		268,885	282,588
Individually evaluated for impairment	4,501		5,218	4,632
Total loans, net of unamortized deferred loan fees and discounts	278,458		276,548	289,647
Construction
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation	2,954		2,497	2,586
Total allowance for loan losses	2,954	2,896	2,497	2,586	2,411	1,826
Loans pooled for evaluation	214,195		183,384	184,302
Individually evaluated for impairment				0
Total loans, net of unamortized deferred loan fees and discounts	214,195		183,384	184,302
Construction | Commercial loans
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation	297		663	960
Total allowance for loan losses	297	358	663	960	977	642
Loans pooled for evaluation	45,161		62,088	69,728
Total loans, net of unamortized deferred loan fees and discounts	45,161		62,088	69,728
Construction | Residential
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation	2,657		1,834	1,626
Total allowance for loan losses	2,657	2,538	1,834	1,626	1,434	1,184
Loans pooled for evaluation	169,034		121,296	114,574
Total loans, net of unamortized deferred loan fees and discounts	169,034		121,296	114,574
Residential 1-4 family | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation	2,315		2,620	2,313
Individually evaluated for impairment	91		56	57
Total allowance for loan losses	2,406	2,576	2,676	2,400	1,991	2,317
Loans pooled for evaluation	499,620		509,267	517,935
Individually evaluated for impairment	4,757		4,321	4,781
Total loans, net of unamortized deferred loan fees and discounts	505,794		515,262	524,414
Home equity loans | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation	1,216		1,486	1,418
Individually evaluated for impairment	46		54	175
Total allowance for loan losses	1,262	1,242	1,540	1,593	1,532	1,841
Loans pooled for evaluation	27,793		33,142	37,840
Individually evaluated for impairment	2,156		3,103	2,406
Total loans, net of unamortized deferred loan fees and discounts	30,370		36,684	40,693
Other | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Loans pooled for evaluation	1,535		779	597
Individually evaluated for impairment	18		14	103
Total allowance for loan losses	1,553	$ 1,451	793	700	$ 557	$ 586
Loans pooled for evaluation	80,146		55,483	49,171
Individually evaluated for impairment	149		196	243
Total loans, net of unamortized deferred loan fees and discounts	80,297		55,721	49,456
Loans acquired with deteriorated credit quality
Loans and Leases Receivable Disclosure [Line Items]
Loans acquired with deteriorated credit quality	6		122	110
Total loans, net of unamortized deferred loan fees and discounts	10,305		14,223	13,798
Loans acquired with deteriorated credit quality | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Loans acquired with deteriorated credit quality			116	89
Total loans, net of unamortized deferred loan fees and discounts	6,546		10,130	9,583
Loans acquired with deteriorated credit quality | Mortgage Loans on Real Estate | Commercial loans
Loans and Leases Receivable Disclosure [Line Items]
Loans acquired with deteriorated credit quality			116	59
Total loans, net of unamortized deferred loan fees and discounts	5,129		8,456	7,885
Loans acquired with deteriorated credit quality | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Loans acquired with deteriorated credit quality	6		6	7
Total loans, net of unamortized deferred loan fees and discounts	1,249		1,648	1,788
Loans acquired with deteriorated credit quality | Consumer | Home equity lines of credit
Loans and Leases Receivable Disclosure [Line Items]
Loans acquired with deteriorated credit quality	6		6	7
Total loans, net of unamortized deferred loan fees and discounts	826		1,167	1,299
Loans acquired with deteriorated credit quality | Commercial
Loans and Leases Receivable Disclosure [Line Items]
Loans acquired with deteriorated credit quality				14
Total loans, net of unamortized deferred loan fees and discounts	2,510		2,445	2,427
Loans acquired with deteriorated credit quality | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts				0
Loans acquired with deteriorated credit quality | Residential 1-4 family | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Loans acquired with deteriorated credit quality				30
Total loans, net of unamortized deferred loan fees and discounts	1,417		1,674	1,698
Loans acquired with deteriorated credit quality | Home equity loans | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	421		439	447
Loans acquired with deteriorated credit quality | Other | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	$ 2		$ 42	$ 42