Loans and Allowance for Loan Losses - Activity in ALL by Class of Loans (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	$ 51,666	$ 48,693
Charge- offs	(599)	(1,881)
Recoveries	169	1,037
Net charge- offs	(430)	(844)
Provisions charged to operations	314	72
Balance at end of period	51,550	47,921
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	6,195	5,602
Charge- offs	(19)	(32)
Recoveries	1	1
Net charge- offs	(18)	(31)
Provisions charged to operations	473	(9)
Balance at end of period	6,650	5,562
Real Estate [Member] | Non-residential Real Estate Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	10,519	10,793
Charge- offs	(1)	(1)
Recoveries	39	1
Net charge- offs	38
Provisions charged to operations	(9)	(5)
Balance at end of period	10,548	10,788
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	3,226	3,129
Charge- offs	(56)	(120)
Recoveries	3	1
Net charge- offs	(53)	(119)
Provisions charged to operations	108	120
Balance at end of period	3,281	3,130
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	9,672	8,622
Charge- offs	(90)	(57)
Recoveries	3	11
Net charge- offs	(87)	(46)
Provisions charged to operations	246	83
Balance at end of period	9,831	8,659
Commercial and Financial [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	15,334	12,421
Charge- offs	(156)	(206)
Recoveries	13	918
Net charge- offs	(143)	712
Provisions charged to operations	(406)	(323)
Balance at end of period	14,785	12,810
Consumer Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,793	2,804
Charge- offs	(250)	(234)
Recoveries	80	51
Net charge- offs	(170)	(183)
Provisions charged to operations	76	104
Balance at end of period	2,699	2,725
Other Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	2,481	4,045
Charge- offs		(1,218)
Recoveries	12	4
Net charge- offs	12	(1,214)
Provisions charged to operations	(157)	127
Balance at end of period	2,336	2,958
Acquired Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period	1,446	1,277
Charge- offs	(27)	(13)
Recoveries	18	50
Net charge- offs	(9)	37
Provisions charged to operations	(17)	(25)
Balance at end of period	$ 1,420	$ 1,289