Allowance for Loan Losses - Summary of Activity in Allowance for Loan Losses, and Ending Balance of Loans, Net of Unearned Fees for Periods Indicated (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	$ 3,069,733	$ 2,761,192	$ 3,015,165
Individ. evaluated for impairment	25,803	27,650	28,234
Loans pooled for evaluation	3,029,454	2,710,005	2,971,371
Beginning balance	30,323	32,503	32,503
Charge-offs	(480)	(409)	(4,410)
Recoveries	366	480	2,141
(Benefit) provision	(236)	(1,557)	89
Ending balance	29,973	31,017	30,323
Ending balance:
Individ. evaluated for impairment	3,091	1,686	2,699
Loans pooled for evaluation	26,714	26,900	27,352
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	309,701	385,426
Individ. evaluated for impairment	5,535	3,849	5,298
Loans pooled for evaluation	378,832	304,493	378,743
Beginning balance	2,317	2,748	2,748
Charge-offs	(1)		(60)
(Benefit) provision	(146)	(86)	(371)
Ending balance	2,170	2,662	2,317
Ending balance:
Individ. evaluated for impairment	190	249	230
Loans pooled for evaluation	1,910	2,188	1,932
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	1,761,114	1,914,896
Individ. evaluated for impairment	11,110	16,979	13,911
Loans pooled for evaluation	1,954,120	1,731,785	1,892,422
Beginning balance	11,441	11,517	11,517
Charge-offs			(186)
Recoveries	15	110	397
(Benefit) provision	39	(85)	(287)
Ending balance	11,495	11,542	11,441
Ending balance:
Individ. evaluated for impairment	154	124	30
Loans pooled for evaluation	11,281	9,971	11,351
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	283,596	288,688
Individ. evaluated for impairment	2,450	2,204	2,688
Loans pooled for evaluation	279,140	277,388	283,502
Beginning balance	5,800	7,044	7,044
Charge-offs	(80)	(71)	(98)
Recoveries	209	46	698
(Benefit) provision	(517)	(489)	(1,844)
Ending balance	5,412	6,530	5,800
Ending balance:
Individ. evaluated for impairment	448	400	427
Loans pooled for evaluation	4,956	6,122	5,356
Consumer and Industrial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	216,015	212,685	220,412
Individ. evaluated for impairment	4,621	3,072	4,470
Loans pooled for evaluation	208,889	205,832	213,358
Beginning balance	6,512	5,831	5,831
Charge-offs	(205)	(133)	(1,444)
Recoveries	50	170	428
(Benefit) provision	35	(542)	1,697
Ending balance	6,392	5,326	6,512
Ending balance:
Individ. evaluated for impairment	2,113	811	1,848
Loans pooled for evaluation	4,249	3,873	4,624
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	59,699	67,930
Individ. evaluated for impairment	136	25	140
Loans pooled for evaluation	71,462	58,830	67,790
Beginning balance	1,184	1,417	1,417
Charge-offs			(1,104)
(Benefit) provision	167	(78)	871
Ending balance	1,351	1,339	1,184
Ending balance:
Individ. evaluated for impairment		14
Loans pooled for evaluation	1,351	1,282	1,184
Construction [Member] | Commercial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	73,952	64,843	69,627
Loans pooled for evaluation	73,952	64,843	69,627
Beginning balance	642	680	680
Recoveries		1	1
(Benefit) provision	205	(109)	(39)
Ending balance	847	572	642
Ending balance:
Loans pooled for evaluation	847	572	642
Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	41,932	40,241	43,031
Individ. evaluated for impairment	1,673	1,241	1,470
Loans pooled for evaluation	39,774	37,867	41,076
Beginning balance	1,841	2,644	2,644
Charge-offs		(31)	(332)
Recoveries	14	12	242
(Benefit) provision	(119)	(174)	(713)
Ending balance	1,736	2,451	1,841
Ending balance:
Individ. evaluated for impairment	130	57	107
Loans pooled for evaluation	1,606	2,328	1,734
Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	23,606	29,313	25,155
Individ. evaluated for impairment	278	280	257
Loans pooled for evaluation	23,285	28,967	24,853
Beginning balance	586	622	622
Charge-offs	(194)	(174)	(1,186)
Recoveries	78	141	375
(Benefit) provision	100	6	775
Ending balance	570	595	586
Ending balance:
Individ. evaluated for impairment	56	31	57
Loans pooled for evaluation	514	564	529
Receivables Acquired with Deteriorated Credit Quality [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	14,476	23,537	15,560
Ending balance:
Loans acquired with deteriorated credit quality	168	2,431	272
Receivables Acquired with Deteriorated Credit Quality [Member] | 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	1,744	1,359	1,385
Ending balance:
Loans acquired with deteriorated credit quality	70	225	155
Receivables Acquired with Deteriorated Credit Quality [Member] | 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	8,038	12,350	8,563
Ending balance:
Loans acquired with deteriorated credit quality	60	1,447	60
Receivables Acquired with Deteriorated Credit Quality [Member] | 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	1,661	4,004	2,498
Ending balance:
Loans acquired with deteriorated credit quality	8	8	17
Receivables Acquired with Deteriorated Credit Quality [Member] | Consumer and Industrial [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	2,505	3,781	2,584
Ending balance:
Loans acquired with deteriorated credit quality	30	642	40
Receivables Acquired with Deteriorated Credit Quality [Member] | Construction [Member] | Residential [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts		844
Ending balance:
Loans acquired with deteriorated credit quality		43
Receivables Acquired with Deteriorated Credit Quality [Member] | Home Equity Loans [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	485	1,133	485
Ending balance:
Loans acquired with deteriorated credit quality		66
Receivables Acquired with Deteriorated Credit Quality [Member] | Other Consumer [Member] | Consumer [Member]
Loans and Leases Receivable Disclosure [Line Items]
Total loans, net of unamortized deferred loan fees and discounts	$ 43	$ 66	$ 45