Loans and Allowance for Loan Losses - Activity in ALL by Class of Loans (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period				$ 48,693				$ 41,666	$ 48,693	$ 41,666
Charge- offs									(6,965)	(4,907)
Recoveries									1,426	415
Net charge- offs									(5,539)	(4,492)
Provisions charged to operations	$ 3,323	$ 3,276	$ 1,841	72	$ 1,672	$ 2,940	$ 2,804	4,103	8,512	11,519	$ 7,675
Balance at end of period	51,666				48,693				51,666	48,693	41,666
Real Estate [Member] | Non-residential Real Estate Owner Occupied [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period				5,602				4,661	5,602	4,661
Charge- offs									(74)	(11)
Recoveries									6	3
Net charge- offs									(68)	(8)
Provisions charged to operations									661	949
Balance at end of period	6,195				5,602				6,195	5,602	4,661
Real Estate [Member] | Non-residential Real Estate Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period				10,793				9,921	10,793	9,921
Charge- offs									(47)	(9)
Recoveries									16	6
Net charge- offs									(31)	(3)
Provisions charged to operations									(243)	875
Balance at end of period	10,519				10,793				10,519	10,793	9,921
Real Estate [Member] | Residential Real Estate Permanent Mortgage [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period				3,129				3,148	3,129	3,148
Charge- offs									(373)	(208)
Recoveries									25	55
Net charge- offs									(348)	(153)
Provisions charged to operations									445	134
Balance at end of period	3,226				3,129				3,226	3,129	3,148
Real Estate [Member] | Residential Real Estate All Other [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period				8,622				6,725	8,622	6,725
Charge- offs									(330)	(181)
Recoveries									66	21
Net charge- offs									(264)	(160)
Provisions charged to operations									1,314	2,057
Balance at end of period	9,672				8,622				9,672	8,622	6,725
Commercial and Financial [Member] | Non-consumer Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period				12,421				11,754	12,421	11,754
Charge- offs									(1,344)	(2,921)
Recoveries									1,033	122
Net charge- offs									(311)	(2,799)
Provisions charged to operations									3,224	3,466
Balance at end of period	15,334				12,421				15,334	12,421	11,754
Consumer Non-real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period				2,804				2,642	2,804	2,642
Charge- offs									(913)	(1,088)
Recoveries									180	159
Net charge- offs									(733)	(929)
Provisions charged to operations									722	1,091
Balance at end of period	2,793				2,804				2,793	2,804	2,642
Other Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period				4,045				2,648	4,045	2,648
Charge- offs									(3,727)	(388)
Recoveries									23	17
Net charge- offs									(3,704)	(371)
Provisions charged to operations									2,140	1,768
Balance at end of period	2,481				4,045				2,481	4,045	2,648
Acquired Loans [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Balance at beginning of period				$ 1,277				$ 167	1,277	167
Charge- offs									(157)	(101)
Recoveries									77	32
Net charge- offs									(80)	(69)
Provisions charged to operations									249	1,179
Balance at end of period	$ 1,446				$ 1,277				$ 1,446	$ 1,277	$ 167