Loans and Leases and Other Real Estate - Age Analysis of Past Due Loans, Segregated by Class of Loans (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 19,008	$ 36,136
Current	4,137,672	3,837,759
Total Loans and Leases	4,156,680	3,873,895
Recorded Investment Greater than 89 Days and Still Accruing	200	10
Commercial, secured by real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	8,562	16,629
Current	2,822,622	2,539,972
Total Loans and Leases	2,831,184	2,556,601
Recorded Investment Greater than 89 Days and Still Accruing	0	0
Commercial, industrial and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	257	1,448
Current	340,143	348,780
Total Loans and Leases	340,400	350,228
Recorded Investment Greater than 89 Days and Still Accruing	0	0
Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	779	363
Current	74,260	66,653
Total Loans and Leases	75,039	67,016
Recorded Investment Greater than 89 Days and Still Accruing	0	0
Real estate - residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	4,984	9,377
Current	317,896	340,204
Total Loans and Leases	322,880	349,581
Recorded Investment Greater than 89 Days and Still Accruing	0	0
Real estate - construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,472	4,881
Current	263,436	206,228
Total Loans and Leases	264,908	211,109
Recorded Investment Greater than 89 Days and Still Accruing	0	0
Home equity and consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,954	3,438
Current	319,315	335,922
Total Loans and Leases	322,269	339,360
Recorded Investment Greater than 89 Days and Still Accruing	200	10
30-59 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	6,436	15,066
30-59 Days Past Due | Commercial, secured by real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,663	6,082
30-59 Days Past Due | Commercial, industrial and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	80	1,193
30-59 Days Past Due | Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	496	132
30-59 Days Past Due | Real estate - residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	939	2,990
30-59 Days Past Due | Real estate - construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	3,409
30-59 Days Past Due | Home equity and consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,258	1,260
60-89 Days Past Due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	2,560	2,711
60-89 Days Past Due | Commercial, secured by real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,082	1,234
60-89 Days Past Due | Commercial, industrial and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	121	213
60-89 Days Past Due | Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	139	78
60-89 Days Past Due | Real estate - residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	908	1,057
60-89 Days Past Due | Real estate - construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	0	0
60-89 Days Past Due | Home equity and consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	310	129
Greater Than 89 Days
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	10,012	18,359
Greater Than 89 Days | Commercial, secured by real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,817	9,313
Greater Than 89 Days | Commercial, industrial and other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	56	42
Greater Than 89 Days | Leases
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	144	153
Greater Than 89 Days | Real estate - residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	3,137	5,330
Greater Than 89 Days | Real estate - construction
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	1,472	1,472
Greater Than 89 Days | Home equity and consumer
Financing Receivable, Recorded Investment, Past Due [Line Items]
Total Past Due	$ 1,386	$ 2,049