Allowance for Loan Losses - Presentation of Credit Quality Indicators (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	$ 3,069,733	$ 3,015,165	$ 2,761,192
Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,755,148	2,690,402
PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	300,109	309,203
PCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	14,476	15,560
Pass [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,688,622	2,617,171
Pass [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	286,976	293,331
Special Mention [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	41,469	38,662
Special Mention [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	9,694	12,408
Substandard [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	25,057	34,569
Substandard [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	3,439	3,464
Mortgage Loans on Real Estate [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,359,379	2,300,322
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	386,111	385,426	309,701
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	323,161	320,522
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	61,206	63,519
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	1,744	1,385
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	317,799	315,120
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	59,184	61,411
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	2,274	2,234
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	214	218
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	3,088	3,168
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	1,808	1,890
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	1,973,268	1,914,896	1,761,114
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	1,754,062	1,690,510
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	211,168	215,823
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	8,038	8,563
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	1,716,599	1,649,333
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	201,641	203,751
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	23,679	18,434
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	8,977	11,513
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	13,784	22,743
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	550	559
Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	348,789	356,874
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	283,251	288,688	283,596
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	266,651	269,942
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	14,939	16,248
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	1,661	2,498
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	263,270	265,345
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	13,686	14,866
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	1,628	2,558
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	282	450
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	1,753	2,039
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	971	932
Consumer [Member] | Home Equity Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	41,932	43,031	40,241
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	38,889	39,848
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	2,558	2,698
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	485
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	35,730	37,428
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	2,301	2,433
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	1,532	800
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	184	188
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	1,627	1,620
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	73	77
Consumer [Member] | Other Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	23,606	25,155	29,313
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	21,422	22,859
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	2,141	2,251
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	43	45
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	21,067	22,432
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	2,067	2,207
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	238	272
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	37	38
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	117	155
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	37	6
Consumer and Industrial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	216,015	220,412	212,685
Consumer and Industrial [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	205,673	209,437
Consumer and Industrial [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	7,837	8,391
Consumer and Industrial [Member] | PCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,505	2,584
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	193,829	195,208
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	7,837	8,390
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	7,263	9,492
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
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	4,581	4,737
Construction [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	145,550	137,557
Construction [Member] | Residential [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	71,598	67,930	59,699
Construction [Member] | Residential [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	71,589	67,920
Construction [Member] | Residential [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	9	10
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	71,482	67,813
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	9	10
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	107
Construction [Member] | Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	73,952	69,627	$ 64,843
Construction [Member] | Commercial [Member] | Originated Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	73,701	69,364
Construction [Member] | Commercial [Member] | PNCI Loans [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	251	263
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	68,846	64,492
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	251	263
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	$ 4,855	$ 4,872