Loans, Leases and Other Real Estate - Allowance for Loan and Lease Losses 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
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans and Leases	$ 4,281,302		$ 4,156,680
Ending Balance: Individually evaluated for impairment, related allowance	865		505
Ending Balance	36,604	$ 35,644	35,455	$ 32,823	$ 31,590	$ 31,245
Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Individually evaluated for impairment	19,304		22,591
Ending Balance: Collectively evaluated for impairment	4,261,312		4,133,379
Total Loans and Leases	4,281,302		4,156,680
Ending Balance: Individually evaluated for impairment, related allowance	865		505
Ending Balance: Collectively evaluated for impairment, related allowance	35,739		34,950
Ending Balance	36,604		35,455
Commercial, secured by real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans and Leases	2,925,104		2,831,184
Ending Balance: Individually evaluated for impairment, related allowance	452		454
Ending Balance	26,174	25,817	25,704	23,344	22,083	21,223
Commercial, secured by real estate | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Individually evaluated for impairment	15,185		17,536
Ending Balance: Collectively evaluated for impairment	2,909,234		2,812,941
Total Loans and Leases	2,925,104		2,831,184
Ending Balance: Individually evaluated for impairment, related allowance	452		454
Ending Balance: Collectively evaluated for impairment, related allowance	25,722		25,250
Ending Balance	26,174		25,704
Commercial, industrial and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans and Leases	339,974		340,400
Ending Balance: Individually evaluated for impairment, related allowance	8		9
Ending Balance	2,012	1,768	2,313	1,688	1,792	1,723
Commercial, industrial and other | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Individually evaluated for impairment	1,753		782
Ending Balance: Collectively evaluated for impairment	338,221		339,618
Total Loans and Leases	339,974		340,400
Ending Balance: Individually evaluated for impairment, related allowance	8		9
Ending Balance: Collectively evaluated for impairment, related allowance	2,004		2,304
Ending Balance	2,012		2,313
Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans and Leases	82,006		75,039
Ending Balance: Individually evaluated for impairment, related allowance	393		30
Ending Balance	1,264	1,042	630	529	502	548
Leases | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Individually evaluated for impairment	663		65
Ending Balance: Collectively evaluated for impairment	81,343		74,974
Total Loans and Leases	82,006		75,039
Ending Balance: Individually evaluated for impairment, related allowance	393		30
Ending Balance: Collectively evaluated for impairment, related allowance	871		600
Ending Balance	1,264		630
Real estate - residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans and Leases	321,717		322,880
Ending Balance: Individually evaluated for impairment, related allowance	4		4
Ending Balance	1,585	1,589	1,557	1,754	1,825	1,964
Real estate - residential mortgage | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Individually evaluated for impairment	757		1,744
Ending Balance: Collectively evaluated for impairment	320,960		321,136
Total Loans and Leases	321,717		322,880
Ending Balance: Individually evaluated for impairment, related allowance	4		4
Ending Balance: Collectively evaluated for impairment, related allowance	1,581		1,553
Ending Balance	1,585		1,557
Real estate - construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans and Leases	297,357		264,908
Ending Balance: Individually evaluated for impairment, related allowance	0		0
Ending Balance	3,063	2,932	2,731	2,596	2,378	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	297,357		263,437
Total Loans and Leases	297,357		264,908
Ending Balance: Individually evaluated for impairment, related allowance	0		0
Ending Balance: Collectively evaluated for impairment, related allowance	3,063		2,731
Ending Balance	3,063		2,731
Home equity and consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Loans and Leases	315,144		322,269
Ending Balance: Individually evaluated for impairment, related allowance	8		8
Ending Balance	2,506	$ 2,496	2,520	$ 2,912	$ 3,010	$ 3,435
Home equity and consumer | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Individually evaluated for impairment	946		993
Ending Balance: Collectively evaluated for impairment	314,197		321,273
Total Loans and Leases	315,144		322,269
Ending Balance: Individually evaluated for impairment, related allowance	8		8
Ending Balance: Collectively evaluated for impairment, related allowance	2,498		2,512
Ending Balance	2,506		2,520
Financial Asset Acquired with Credit Deterioration | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Loans acquired with deteriorated credit quality	686		710
Financial Asset Acquired with Credit Deterioration | Commercial, secured by real estate | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Loans acquired with deteriorated credit quality	685		707
Financial Asset Acquired with Credit Deterioration | Commercial, industrial and other | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Loans acquired with deteriorated credit quality	0		0
Financial Asset Acquired with Credit Deterioration | Leases | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Loans acquired with deteriorated credit quality	0		0
Financial Asset Acquired with Credit Deterioration | Real estate - residential mortgage | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Loans acquired with deteriorated credit quality	0		0
Financial Asset Acquired with Credit Deterioration | Real estate - construction | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Loans acquired with deteriorated credit quality	0		0
Financial Asset Acquired with Credit Deterioration | Home equity and consumer | Loans and Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Ending Balance: Loans acquired with deteriorated credit quality	$ 1		$ 3