Allowance for Loan Losses - Schedule Credit Quality Indicators (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Loans and Leases Receivable Disclosure [Line Items]
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]
Total loans, net of unamortized deferred loan fees and discounts	3,247,156	3,143,100	3,132,202
Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	442,539	418,982	421,285
Commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	278,458	276,548	289,647
Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	214,195	183,384	184,302
Residential 1-4 family | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
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]
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]
Total loans, net of unamortized deferred loan fees and discounts	80,297	55,721	49,456
Originated
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	3,310,543	2,981,456
Originated | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,464,955	2,254,777
Originated | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	385,598	351,133
Originated | Commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	256,379	228,635
Originated | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	203,611	146,911
Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	355,646	343,796
Originated | Home equity loans | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	27,034	32,660
Originated | Other | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	64,153	34,020
PNCI
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	861,500	1,026,335
PNCI | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	775,655	878,193
PNCI | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	55,692	66,201
PNCI | Commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	19,569	45,468
PNCI | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	10,584	36,473
PNCI | Residential 1-4 family | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	148,731	169,792
PNCI | Home equity loans | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,915	3,585
PNCI | Other | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	16,142	21,659
Commercial loans | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,741,362	2,627,838	2,607,788
Commercial loans | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	45,161	62,088	69,728
Commercial loans | Originated | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,109,309	1,910,981
Commercial loans | Originated | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	44,704	56,208
Commercial loans | PNCI | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	626,924	708,401
Commercial loans | PNCI | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	457	5,880
Home equity lines of credit | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	331,872	326,577	331,136
Home equity lines of credit | Originated | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	294,411	284,453
Home equity lines of credit | PNCI | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	36,635	40,957
Residential | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	169,034	121,296	$ 114,574
Residential | Originated | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	158,907	90,703
Residential | PNCI | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	10,127	30,593
Pass | Originated
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	3,234,025	2,904,603
Pass | Originated | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,412,659	2,198,816
Pass | Originated | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	376,021	342,386
Pass | Originated | Commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	242,308	217,126
Pass | Originated | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	203,037	146,275
Pass | Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	348,668	337,189
Pass | Originated | Home equity loans | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	23,819	29,289
Pass | Originated | Other | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	63,648	33,606
Pass | PNCI
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	842,642	1,014,950
Pass | PNCI | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	763,513	869,776
Pass | PNCI | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	53,293	63,674
Pass | PNCI | Commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	19,399	45,027
Pass | PNCI | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	6,437	36,473
Pass | PNCI | Residential 1-4 family | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	145,141	167,908
Pass | PNCI | Home equity loans | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,841	3,413
Pass | PNCI | Other | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	15,792	21,481
Pass | Commercial loans | Originated | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,063,991	1,861,627
Pass | Commercial loans | Originated | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	44,380	55,863
Pass | Commercial loans | PNCI | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	618,372	701,868
Pass | Commercial loans | PNCI | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	457	5,880
Pass | Home equity lines of credit | Originated | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	288,554	279,491
Pass | Home equity lines of credit | PNCI | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	34,660	38,780
Pass | Residential | Originated | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	158,657	90,412
Pass | Residential | PNCI | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	5,980	30,593
Special Mention | Originated
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	55,199	45,415
Special Mention | Originated | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	40,893	35,207
Special Mention | Originated | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	5,014	3,704
Special Mention | Originated | Commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	8,968	6,127
Special Mention | Originated | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	324	377
Special Mention | Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,743	1,724
Special Mention | Originated | Home equity loans | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,184	1,054
Special Mention | Originated | Other | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	452	341
Special Mention | PNCI
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	9,671	5,863
Special Mention | PNCI | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	4,291	4,171
Special Mention | PNCI | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,232	1,371
Special Mention | PNCI | Commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1	321
Special Mention | PNCI | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	4,147
Special Mention | PNCI | Residential 1-4 family | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,617	1,086
Special Mention | PNCI | Home equity loans | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	0	74
Special Mention | PNCI | Other | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	341	173
Special Mention | Commercial loans | Originated | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	38,150	33,483
Special Mention | Commercial loans | Originated | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	324	345
Special Mention | Commercial loans | PNCI | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,674	3,085
Special Mention | Home equity lines of credit | Originated | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	3,378	2,309
Special Mention | Home equity lines of credit | PNCI | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	891	1,124
Special Mention | Residential | Originated | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts		32
Special Mention | Residential | PNCI | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	4,147
Substandard | Originated
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	21,319	31,438
Substandard | Originated | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	11,403	20,754
Substandard | Originated | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	4,563	5,043
Substandard | Originated | Commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	5,103	5,382
Substandard | Originated | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	250	259
Substandard | Originated | Residential 1-4 family | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	4,235	4,883
Substandard | Originated | Home equity loans | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,031	2,317
Substandard | Originated | Other | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	53	73
Substandard | PNCI
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	9,187	5,522
Substandard | PNCI | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	7,851	4,246
Substandard | PNCI | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,167	1,156
Substandard | PNCI | Commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	169	120
Substandard | PNCI | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	0
Substandard | PNCI | Residential 1-4 family | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,973	798
Substandard | PNCI | Home equity loans | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	74	98
Substandard | PNCI | Other | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	9	5
Substandard | Commercial loans | Originated | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	7,168	15,871
Substandard | Commercial loans | PNCI | Mortgage Loans on Real Estate
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	5,878	3,448
Substandard | Commercial loans | PNCI | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	0
Substandard | Home equity lines of credit | Originated | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,479	2,653
Substandard | Home equity lines of credit | PNCI | Consumer
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,084	1,053
Substandard | Residential | Originated | Construction
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	250	$ 259
Doubtful / Loss | Originated
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	0
Doubtful / Loss | Originated | Commercial
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts