Loans and Allowance for Loan Losses - Activity in ALL by Class of Loans (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	$ 52,200	$ 49,005	$ 51,666	$ 48,693
Charge- offs	(1,271)	(1,138)	(2,542)	(3,898)
Recoveries	199	112	465	1,271
Net charge- offs	(1,072)	(1,026)	(2,077)	(2,627)
Provisions charged to operations	747	3,276	2,286	5,189
Balance at end of period	51,875	51,255	51,875	51,255
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	6,426	5,685	6,195	5,602
Charge- offs	(179)	(2)	(198)	(74)
Recoveries	15	1	16	5
Net charge- offs	(164)	(1)	(182)	(69)
Provisions charged to operations	164	261	413	412
Balance at end of period	6,426	5,945	6,426	5,945
Real Estate [Member] | Non-residential Real Estate Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	10,705	10,480	10,519	10,793
Charge- offs	(8)		(9)	(26)
Recoveries		1	39	3
Net charge- offs	(8)	1	30	(23)
Provisions charged to operations	(49)	70	99	(219)
Balance at end of period	10,648	10,551	10,648	10,551
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,307	3,148	3,226	3,129
Charge- offs	(39)	(90)	(101)	(246)
Recoveries		9	26	20
Net charge- offs	(39)	(81)	(75)	(226)
Provisions charged to operations	43	91	160	255
Balance at end of period	3,311	3,158	3,311	3,158
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	10,123	8,912	9,672	8,622
Charge- offs	(71)	(89)	(312)	(162)
Recoveries	95	13	101	30
Net charge- offs	24	(76)	(211)	(132)
Provisions charged to operations	274	503	960	849
Balance at end of period	10,421	9,339	10,421	9,339
Commercial and Financial [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	15,069	13,643	15,334	12,421
Charge- offs	(343)	(538)	(652)	(1,215)
Recoveries	7	24	30	986
Net charge- offs	(336)	(514)	(622)	(229)
Provisions charged to operations	(337)	1,838	(316)	2,775
Balance at end of period	14,396	14,967	14,396	14,967
Consumer Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,839	2,706	2,793	2,804
Charge- offs	(294)	(238)	(738)	(706)
Recoveries	70	57	194	140
Net charge- offs	(224)	(181)	(544)	(566)
Provisions charged to operations	298	207	664	494
Balance at end of period	2,913	2,732	2,913	2,732
Other Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,328	3,006	2,481	4,045
Charge- offs		(47)	(2)	(1,321)
Recoveries	6		30	22
Net charge- offs	6	(47)	28	(1,299)
Provisions charged to operations	88	153	(87)	366
Balance at end of period	2,422	3,112	2,422	3,112
Acquired Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	1,403	1,425	1,446	1,277
Charge- offs	(337)	(134)	(530)	(148)
Recoveries	6	7	29	65
Net charge- offs	(331)	(127)	(501)	(83)
Provisions charged to operations	266	153	393	257
Balance at end of period	$ 1,338	$ 1,451	$ 1,338	$ 1,451