Loans, Leases and Other Real Estate - Allowance for Loan and Lease Losses by Portfolio Segment (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans and Leases	$ 4,332,238		$ 4,156,680
Ending Balance: Individually evaluated for impairment, related allowance	349		505
Ending Balance	37,293	$ 36,604	35,455	$ 33,925	$ 32,823	$ 31,245
Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Individually evaluated for impairment	18,380		22,591
Ending Balance: Collectively evaluated for impairment	4,313,185		4,133,379
Ending Balance: Loans acquired with deteriorated credit quality	673		710
Total Loans and Leases	4,332,238		4,156,680
Ending Balance: Individually evaluated for impairment, related allowance	349		505
Ending Balance: Collectively evaluated for impairment, related allowance	36,944		34,950
Ending Balance	37,293		35,455
Commercial, secured by real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans and Leases	2,984,430		2,831,184
Ending Balance: Individually evaluated for impairment, related allowance	322		454
Ending Balance	27,646	26,174	25,704	24,728	23,344	21,223
Commercial, secured by real estate | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Individually evaluated for impairment	14,900		17,536
Ending Balance: Collectively evaluated for impairment	2,968,859		2,812,941
Ending Balance: Loans acquired with deteriorated credit quality	671		707
Total Loans and Leases	2,984,430		2,831,184
Ending Balance: Individually evaluated for impairment, related allowance	322		454
Ending Balance: Collectively evaluated for impairment, related allowance	27,324		25,250
Ending Balance	27,646		25,704
Commercial, industrial and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans and Leases	334,241		340,400
Ending Balance: Individually evaluated for impairment, related allowance	8		9
Ending Balance	1,968	2,012	2,313	2,092	1,688	1,723
Commercial, industrial and other | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Individually evaluated for impairment	1,548		782
Ending Balance: Collectively evaluated for impairment	332,693		339,618
Ending Balance: Loans acquired with deteriorated credit quality	0		0
Total Loans and Leases	334,241		340,400
Ending Balance: Individually evaluated for impairment, related allowance	8		9
Ending Balance: Collectively evaluated for impairment, related allowance	1,960		2,304
Ending Balance	1,968		2,313
Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans and Leases	82,881		75,039
Ending Balance: Individually evaluated for impairment, related allowance	8		30
Ending Balance	886	1,264	630	514	529	548
Leases | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Individually evaluated for impairment	318		65
Ending Balance: Collectively evaluated for impairment	82,563		74,974
Ending Balance: Loans acquired with deteriorated credit quality	0		0
Total Loans and Leases	82,881		75,039
Ending Balance: Individually evaluated for impairment, related allowance	8		30
Ending Balance: Collectively evaluated for impairment, related allowance	878		600
Ending Balance	886		630
Real estate - residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans and Leases	315,135		322,880
Ending Balance: Individually evaluated for impairment, related allowance	4		4
Ending Balance	1,487	1,585	1,557	1,569	1,754	1,964
Real estate - residential mortgage | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Individually evaluated for impairment	743		1,744
Ending Balance: Collectively evaluated for impairment	314,392		321,136
Ending Balance: Loans acquired with deteriorated credit quality	0		0
Total Loans and Leases	315,135		322,880
Ending Balance: Individually evaluated for impairment, related allowance	4		4
Ending Balance: Collectively evaluated for impairment, related allowance	1,483		1,553
Ending Balance	1,487		1,557
Real estate - construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans and Leases	297,516		264,908
Ending Balance: Individually evaluated for impairment, related allowance	0		0
Ending Balance	2,901	3,063	2,731	2,465	2,596	2,352
Real estate - construction | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Individually evaluated for impairment	0		1,471
Ending Balance: Collectively evaluated for impairment	297,516		263,437
Ending Balance: Loans acquired with deteriorated credit quality	0		0
Total Loans and Leases	297,516		264,908
Ending Balance: Individually evaluated for impairment, related allowance	0		0
Ending Balance: Collectively evaluated for impairment, related allowance	2,901		2,731
Ending Balance	2,901		2,731
Home equity and consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans and Leases	318,035		322,269
Ending Balance: Individually evaluated for impairment, related allowance	7		8
Ending Balance	2,405	$ 2,506	2,520	$ 2,557	$ 2,912	$ 3,435
Home equity and consumer | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Individually evaluated for impairment	871		993
Ending Balance: Collectively evaluated for impairment	317,162		321,273
Ending Balance: Loans acquired with deteriorated credit quality	2		3
Total Loans and Leases	318,035		322,269
Ending Balance: Individually evaluated for impairment, related allowance	7		8
Ending Balance: Collectively evaluated for impairment, related allowance	2,398		2,512
Ending Balance	$ 2,405		$ 2,520