Allowance for Loan Losses - Schedule Credit Quality Indicators (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	$ 4,034,331	$ 4,022,014	$ 3,069,733
Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	3,129,339	3,143,100	2,359,379
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	418,352	418,982	348,789
Commercial Portfolio Segment [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	269,163	276,548	216,015
Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	217,477	183,384	145,550
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	522,412	515,262	386,111
Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	35,037	36,684	41,932
Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	65,062	55,721	23,606
Originated [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	3,049,985	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	2,287,067	2,254,777
Originated [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	355,340	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	227,314	228,635
Originated [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	180,264	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	357,559	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	31,245	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	45,020	34,020
PNCI [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	972,620	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	834,665	878,193
PNCI [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	61,447	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	39,295	45,468
PNCI [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	37,213	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	163,268	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	3,356	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	20,001	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	2,606,927	2,627,838	1,973,268
Commercial [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	71,693	62,088	73,952
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	1,929,508	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	64,576	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	671,397	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	7,117	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	318,253	326,577	283,251
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	279,075	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	38,090	40,957
Residential [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	145,784	121,296	$ 71,598
Residential [Member] | Originated [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	115,688	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	30,096	30,593
Pass [Member] | Originated [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,976,725	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	2,237,214	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	345,083	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	214,758	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	179,670	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	350,256	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	27,328	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	44,611	33,606
Pass [Member] | PNCI [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	961,563	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	826,981	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	58,852	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	38,762	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	36,968	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	161,351	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	3,174	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	19,790	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	1,886,958	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	64,238	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	665,630	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	6,872	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	273,144	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	35,888	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	115,432	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	30,096	30,593
Special Mention [Member] | Originated [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	47,844	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	34,901	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	4,709	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	7,896	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	338	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	1,807	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	1,533	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	309	341
Special Mention [Member] | PNCI [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	5,268	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	3,836	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	1,231	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	201	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	1,109	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	98	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	208	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	33,094	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	338	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	2,727	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	2,867	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	925	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	25,416	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	14,952	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	5,548	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	4,660	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	256	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	5,496	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	2,384	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	100	73
Substandard [Member] | PNCI [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	5,789	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	3,848	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	1,364	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	332	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	808	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	84	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	9,456	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	3,040	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	3,064	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	1,277	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	256	$ 259
Substandard [Member] | Residential [Member] | PNCI [Member] | Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	$ 0