Allowance for Loan Losses - Presentation of Credit Quality Indicators (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	$ 3,015,165	$ 2,759,593	$ 2,522,937
Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,690,402	2,356,327
PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	309,203	378,222
PCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	15,560	25,044
Pass [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,617,171	2,305,293
Pass [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	293,331	360,043
Special Mention [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	38,662	27,571
Special Mention [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	12,408	8,549
Substandard [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	34,569	23,463
Substandard [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	3,464	9,630
Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,300,322	2,053,464
Mortgage Loans on Real Estate [Member] | Residential [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	385,426	368,343	314,265
Mortgage Loans on Real Estate [Member] | Residential [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	320,522	284,539
Mortgage Loans on Real Estate [Member] | Residential [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	63,519	82,335
Mortgage Loans on Real Estate [Member] | Residential [Member] | PCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,385	1,469
Mortgage Loans on Real Estate [Member] | Residential [Member] | Pass [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	315,120	278,635
Mortgage Loans on Real Estate [Member] | Residential [Member] | Pass [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	61,411	79,000
Mortgage Loans on Real Estate [Member] | Residential [Member] | Special Mention [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,234	2,992
Mortgage Loans on Real Estate [Member] | Residential [Member] | Special Mention [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	218	1,849
Mortgage Loans on Real Estate [Member] | Residential [Member] | Substandard [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	3,168	2,912
Mortgage Loans on Real Estate [Member] | Residential [Member] | Substandard [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,890	1,486
Mortgage Loans on Real Estate [Member] | Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,914,896	1,685,121	1,497,567
Mortgage Loans on Real Estate [Member] | Commercial [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,690,510	1,425,828
Mortgage Loans on Real Estate [Member] | Commercial [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	215,823	246,491
Mortgage Loans on Real Estate [Member] | Commercial [Member] | PCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	8,563	12,802
Mortgage Loans on Real Estate [Member] | Commercial [Member] | Pass [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,649,333	1,399,936
Mortgage Loans on Real Estate [Member] | Commercial [Member] | Pass [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	203,751	233,326
Mortgage Loans on Real Estate [Member] | Commercial [Member] | Special Mention [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	18,434	14,341
Mortgage Loans on Real Estate [Member] | Commercial [Member] | Special Mention [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	11,513	5,925
Mortgage Loans on Real Estate [Member] | Commercial [Member] | Substandard [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	22,743	11,551
Mortgage Loans on Real Estate [Member] | Commercial [Member] | Substandard [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	559	7,240
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	356,874	366,663
Consumer [Member] | Home Equity Lines of Credit [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	288,688	289,715	322,492
Consumer [Member] | Home Equity Lines of Credit [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	269,942	263,590
Consumer [Member] | Home Equity Lines of Credit [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	16,248	21,765
Consumer [Member] | Home Equity Lines of Credit [Member] | PCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,498	4,360
Consumer [Member] | Home Equity Lines of Credit [Member] | Pass [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	265,345	258,024
Consumer [Member] | Home Equity Lines of Credit [Member] | Pass [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	14,866	20,442
Consumer [Member] | Home Equity Lines of Credit [Member] | Special Mention [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,558	2,518
Consumer [Member] | Home Equity Lines of Credit [Member] | Special Mention [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	450	509
Consumer [Member] | Home Equity Lines of Credit [Member] | Substandard [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,039	3,048
Consumer [Member] | Home Equity Lines of Credit [Member] | Substandard [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	932	814
Consumer [Member] | Home Equity Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	43,031	46,182	40,362
Consumer [Member] | Home Equity Loans [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	39,848	40,736
Consumer [Member] | Home Equity Loans [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,698	3,764
Consumer [Member] | Home Equity Loans [Member] | PCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	485	1,682
Consumer [Member] | Home Equity Loans [Member] | Pass [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	37,428	37,844
Consumer [Member] | Home Equity Loans [Member] | Pass [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,433	3,506
Consumer [Member] | Home Equity Loans [Member] | Special Mention [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	800	891
Consumer [Member] | Home Equity Loans [Member] | Special Mention [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	188	173
Consumer [Member] | Home Equity Loans [Member] | Substandard [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1,620	2,001
Consumer [Member] | Home Equity Loans [Member] | Substandard [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	77	85
Consumer [Member] | Other Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	25,155	30,766	32,429
Consumer [Member] | Other Consumer [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	22,859	28,167
Consumer [Member] | Other Consumer [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,251	2,534
Consumer [Member] | Other Consumer [Member] | PCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	45	65
Consumer [Member] | Other Consumer [Member] | Pass [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	22,432	27,542
Consumer [Member] | Other Consumer [Member] | Pass [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,207	2,437
Consumer [Member] | Other Consumer [Member] | Special Mention [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	272	385
Consumer [Member] | Other Consumer [Member] | Special Mention [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	38	92
Consumer [Member] | Other Consumer [Member] | Substandard [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	155	240
Consumer [Member] | Other Consumer [Member] | Substandard [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	6	5
Consumer and Industrial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	220,412	217,047	194,913
Consumer and Industrial [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	209,437	200,735
Consumer and Industrial [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	8,391	12,321
Consumer and Industrial [Member] | PCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,584	3,991
Consumer and Industrial [Member] | Pass [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	195,208	190,902
Consumer and Industrial [Member] | Pass [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	8,390	12,320
Consumer and Industrial [Member] | Special Mention [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	9,492	6,133
Consumer and Industrial [Member] | Special Mention [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	1	1
Consumer and Industrial [Member] | Substandard [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	4,737	3,700
Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	137,557	122,419
Construction [Member] | Residential [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	67,930	55,429	46,135
Construction [Member] | Residential [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	67,920	54,613
Construction [Member] | Residential [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	10	141
Construction [Member] | Residential [Member] | PCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts		675
Construction [Member] | Residential [Member] | Pass [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	67,813	54,602
Construction [Member] | Residential [Member] | Pass [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	10	141
Construction [Member] | Residential [Member] | Substandard [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	107	11
Construction [Member] | Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	69,627	66,990	$ 74,774
Construction [Member] | Commercial [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	69,364	58,119
Construction [Member] | Commercial [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	263	8,871
Construction [Member] | Commercial [Member] | Pass [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	64,492	57,808
Construction [Member] | Commercial [Member] | Pass [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	263	8,871
Construction [Member] | Commercial [Member] | Special Mention [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	$ 4,872	$ 311