Loans - Allowance for Loan Losses and Recorded Investment in Loans by Portfolio Segment (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Ending allowance balance attributable to loans:
Individually evaluated for impairment	$ 763		$ 47
Collectively evaluated for impairment	14,836		13,957
Modified in a troubled debt restructuring	6,523		5,689
Ending allowance balance	22,122	$ 20,756	19,693	$ 17,269	$ 16,546	$ 16,330
Loans:
Individually evaluated for impairment	8,732		1,238
Collectively evaluated for impairment	2,312,328		2,130,228
Acquired with deteriorated credit quality	1,511,008		1,349,203
Modified in a troubled debt restructuring	18,002		17,303
Total ending loans balance	2,339,649		2,149,848
Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Ending allowance balance	0		0
Loans:
Acquired with deteriorated credit quality	587		1,079
Commercial, Industrial, and Agricultural
Ending allowance balance attributable to loans:
Individually evaluated for impairment	762		47
Collectively evaluated for impairment	6,124		5,868
Modified in a troubled debt restructuring	257		245
Ending allowance balance	7,143	6,282	6,160	5,563	4,785	5,428
Loans:
Individually evaluated for impairment	6,887		1,187
Collectively evaluated for impairment	796,806		742,738
Acquired with deteriorated credit quality	808,565		749,138
Modified in a troubled debt restructuring	4,872		5,213
Total ending loans balance	808,565		749,138
Commercial, Industrial, and Agricultural | Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Ending allowance balance	0		0
Loans:
Acquired with deteriorated credit quality	0		0
Commercial mortgages
Ending allowance balance attributable to loans:
Individually evaluated for impairment	1		0
Collectively evaluated for impairment	4,348		3,563
Modified in a troubled debt restructuring	6,266		5,444
Ending allowance balance	10,615	10,020	9,007	7,641	7,357	6,753
Loans:
Individually evaluated for impairment	1,845		51
Collectively evaluated for impairment	686,881		586,845
Acquired with deteriorated credit quality	702,443		600,065
Modified in a troubled debt restructuring	13,130		12,090
Total ending loans balance	702,443		600,065
Commercial mortgages | Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Ending allowance balance	0		0
Loans:
Acquired with deteriorated credit quality	587		1,079
Residential Real Estate
Ending allowance balance attributable to loans:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	1,900		2,033
Modified in a troubled debt restructuring	0		0
Ending allowance balance	1,900	2,052	2,033	1,670	2,022	1,653
Loans:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	738,839		713,347
Modified in a troubled debt restructuring	0		0
Total ending loans balance	738,839		713,347
Residential Real Estate | Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Ending allowance balance	0		0
Loans:
Acquired with deteriorated credit quality	0		0
Consumer
Ending allowance balance attributable to loans:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	2,156		2,179
Modified in a troubled debt restructuring	0		0
Ending allowance balance	2,156	2,065	2,179	2,068	2,089	2,215
Loans:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	82,089		80,193
Modified in a troubled debt restructuring	0		0
Total ending loans balance	82,089		80,193
Consumer | Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Ending allowance balance	0		0
Loans:
Acquired with deteriorated credit quality	0		0
Credit Cards
Ending allowance balance attributable to loans:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	101		120
Modified in a troubled debt restructuring	0		0
Ending allowance balance	101	123	120	142	105	93
Loans:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	7,414		6,753
Modified in a troubled debt restructuring	0		0
Total ending loans balance	7,414		6,753
Credit Cards | Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Ending allowance balance	0		0
Loans:
Acquired with deteriorated credit quality	0		0
Overdrafts
Ending allowance balance attributable to loans:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	207		194
Modified in a troubled debt restructuring	0		0
Ending allowance balance	207	$ 214	194	$ 185	$ 188	$ 188
Loans:
Individually evaluated for impairment	0		0
Collectively evaluated for impairment	299		352
Modified in a troubled debt restructuring	0		0
Total ending loans balance	299		352
Overdrafts | Acquired with deteriorated credit quality
Ending allowance balance attributable to loans:
Ending allowance balance	0		0
Loans:
Acquired with deteriorated credit quality	$ 0		$ 0