Allowance for Loan Losses - Schedule Credit Quality Indicators (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	$ 4,103,687	$ 4,022,014	$ 3,146,313
Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	3,178,730	3,143,100	2,401,040
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	434,388	418,982	350,925
Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	276,045	276,548	237,619
Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	214,524	183,384	156,729
Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	506,049	515,262	384,692
Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	32,345	36,684	40,986
Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	71,143	55,721	23,503
Originated [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	3,184,888	2,981,456
Originated [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,354,722	2,254,777
Originated [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	376,922	351,133
Originated [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	248,523	228,635
Originated [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	204,721	146,911
Originated [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	348,737	343,796
Originated [Member] | Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	29,041	32,660
Originated [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	53,340	34,020
PNCI [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	907,304	1,026,335
PNCI [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	816,609	878,193
PNCI [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	55,908	66,201
PNCI [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	24,984	45,468
PNCI [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	9,803	36,473
PNCI [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	155,872	169,792
PNCI [Member] | Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,875	3,585
PNCI [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	17,802	21,659
Commercial [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,672,681	2,627,838	2,016,348
Commercial [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	57,009	62,088	83,151
Commercial [Member] | Originated [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,005,985	1,910,981
Commercial [Member] | Originated [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	56,289	56,208
Commercial [Member] | PNCI [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	660,737	708,401
Commercial [Member] | PNCI [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	720	5,880
Home Equity Lines of Credit [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	330,900	326,577	286,436
Home Equity Lines of Credit [Member] | Originated [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	294,541	284,453
Home Equity Lines of Credit [Member] | PNCI [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	35,231	40,957
Residential [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	157,515	121,296	$ 73,578
Residential [Member] | Originated [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	148,432	90,703
Residential [Member] | PNCI [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	9,083	30,593
Pass [Member] | Originated [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	3,118,133	2,904,603
Pass [Member] | Originated [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,307,139	2,198,816
Pass [Member] | Originated [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	369,833	342,386
Pass [Member] | Originated [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	237,025	217,126
Pass [Member] | Originated [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	204,136	146,275
Pass [Member] | Originated [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	342,847	337,189
Pass [Member] | Originated [Member] | Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	26,265	29,289
Pass [Member] | Originated [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	53,044	33,606
Pass [Member] | PNCI [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	892,884	1,014,950
Pass [Member] | PNCI [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	805,059	869,776
Pass [Member] | PNCI [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	53,617	63,674
Pass [Member] | PNCI [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	24,650	45,027
Pass [Member] | PNCI [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	9,558	36,473
Pass [Member] | PNCI [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	154,238	167,908
Pass [Member] | PNCI [Member] | Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,727	3,413
Pass [Member] | PNCI [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	17,545	21,481
Pass [Member] | Commercial [Member] | Originated [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,964,292	1,861,627
Pass [Member] | Commercial [Member] | Originated [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	55,958	55,863
Pass [Member] | Commercial [Member] | PNCI [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	650,821	701,868
Pass [Member] | Commercial [Member] | PNCI [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	475	5,880
Pass [Member] | Home Equity Lines of Credit [Member] | Originated [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	290,524	279,491
Pass [Member] | Home Equity Lines of Credit [Member] | PNCI [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	33,345	38,780
Pass [Member] | Residential [Member] | Originated [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	148,178	90,412
Pass [Member] | Residential [Member] | PNCI [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	9,083	30,593
Special Mention [Member] | Originated [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	41,561	45,415
Special Mention [Member] | Originated [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	33,610	35,207
Special Mention [Member] | Originated [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,426	3,704
Special Mention [Member] | Originated [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	5,194	6,127
Special Mention [Member] | Originated [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	331	377
Special Mention [Member] | Originated [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,048	1,724
Special Mention [Member] | Originated [Member] | Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	564	1,054
Special Mention [Member] | Originated [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	189	341
Special Mention [Member] | PNCI [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	5,124	5,863
Special Mention [Member] | PNCI [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	4,005	4,171
Special Mention [Member] | PNCI [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,118	1,371
Special Mention [Member] | PNCI [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1	321
Special Mention [Member] | PNCI [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	864	1,086
Special Mention [Member] | PNCI [Member] | Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	69	74
Special Mention [Member] | PNCI [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	254	173
Special Mention [Member] | Commercial [Member] | Originated [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	32,562	33,483
Special Mention [Member] | Commercial [Member] | Originated [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	331	345
Special Mention [Member] | Commercial [Member] | PNCI [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	3,141	3,085
Special Mention [Member] | Home Equity Lines of Credit [Member] | Originated [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,673	2,309
Special Mention [Member] | Home Equity Lines of Credit [Member] | PNCI [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	795	1,124
Special Mention [Member] | Residential [Member] | Originated [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts		32
Substandard [Member] | Originated [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	24,883	31,438
Substandard [Member] | Originated [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	13,973	20,754
Substandard [Member] | Originated [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	4,663	5,043
Substandard [Member] | Originated [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	5,993	5,382
Substandard [Member] | Originated [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	254	259
Substandard [Member] | Originated [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	4,842	4,883
Substandard [Member] | Originated [Member] | Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,212	2,317
Substandard [Member] | Originated [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	107	73
Substandard [Member] | PNCI [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	9,296	5,522
Substandard [Member] | PNCI [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	7,545	4,246
Substandard [Member] | PNCI [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,173	1,156
Substandard [Member] | PNCI [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	333	120
Substandard [Member] | PNCI [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	245
Substandard [Member] | PNCI [Member] | Residential 1-4 Family [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	770	798
Substandard [Member] | PNCI [Member] | Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	79	98
Substandard [Member] | PNCI [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	3	5
Substandard [Member] | Commercial [Member] | Originated [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	9,131	15,871
Substandard [Member] | Commercial [Member] | PNCI [Member] | Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	6,775	3,448
Substandard [Member] | Commercial [Member] | PNCI [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	245
Substandard [Member] | Home Equity Lines of Credit [Member] | Originated [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,344	2,653
Substandard [Member] | Home Equity Lines of Credit [Member] | PNCI [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,091	1,053
Substandard [Member] | Residential [Member] | Originated [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	254	$ 259
Doubtful [Member] | Originated [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	311
Doubtful [Member] | Originated [Member] | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	$ 311