Loans and Leases and Other Real Estate - Impaired Loans with and without Specific Allowances (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Impaired [Line Items]
Recorded Investment in Impaired Loans	$ 22,591	$ 25,285	$ 26,006
Contractual Unpaid Principal Balance	23,461	27,356	27,621
Loans with related allowance, Related Allowance	505	529
Related Allowance	505	529	843
Interest Income Recognized	705	648	770
Average Investment in Impaired Loans	23,236	26,675	25,749
Commercial, secured by real estate
Financing Receivable, Impaired [Line Items]
Loans without related allowance, Recorded Investment in Impaired Loans	12,155	12,764	14,065
Loans with related allowance, Recorded Investment in Impaired Loans	5,381	5,860	5,721
Recorded Investment in Impaired Loans	17,536	18,624	19,786
Loans without related allowance, Contractual Unpaid Principal Balance	12,497	13,195	14,712
Loans with related allowance, Contractual Unpaid Principal Balance	5,721	6,142	5,918
Contractual Unpaid Principal Balance	18,218	19,337	20,630
Loans with related allowance, Related Allowance	454	392	598
Related Allowance	454	392	598
Loans without related allowance, Interest Income Recognized	366	229	344
Loans with related allowance, Interest Income Recognized	206	273	271
Interest Income Recognized	572	502	615
Loans without related allowance, Average Investment in Impaired Loans	12,774	13,631	12,928
Loans with related allowance, Average Investment in Impaired Loans	5,029	6,549	6,249
Average Investment in Impaired Loans	17,803	20,180	19,177
Commercial, industrial and other
Financing Receivable, Impaired [Line Items]
Loans without related allowance, Recorded Investment in Impaired Loans	618	603	209
Loans with related allowance, Recorded Investment in Impaired Loans	164	349	1,023
Recorded Investment in Impaired Loans	782	952	1,232
Loans without related allowance, Contractual Unpaid Principal Balance	618	603	887
Loans with related allowance, Contractual Unpaid Principal Balance	164	349	1,023
Contractual Unpaid Principal Balance	782	952	1,910
Loans with related allowance, Related Allowance	9	12	77
Related Allowance	9	12	77
Loans without related allowance, Interest Income Recognized	25	24	14
Loans with related allowance, Interest Income Recognized	14	17	32
Interest Income Recognized	39	41	46
Loans without related allowance, Average Investment in Impaired Loans	618	1,109	749
Loans with related allowance, Average Investment in Impaired Loans	283	360	717
Average Investment in Impaired Loans	901	1,469	1,466
Leases
Financing Receivable, Impaired [Line Items]
Loans without related allowance, Recorded Investment in Impaired Loans	0	0	0
Loans with related allowance, Recorded Investment in Impaired Loans	65	0	6
Recorded Investment in Impaired Loans	65	0	6
Loans without related allowance, Contractual Unpaid Principal Balance	0	0	0
Loans with related allowance, Contractual Unpaid Principal Balance	65	0	6
Contractual Unpaid Principal Balance	65	0	6
Loans with related allowance, Related Allowance	30	0	1
Related Allowance	30	0	1
Loans without related allowance, Interest Income Recognized	0	0	0
Loans with related allowance, Interest Income Recognized	0	0	0
Interest Income Recognized	0	0	0
Loans without related allowance, Average Investment in Impaired Loans	0	0	0
Loans with related allowance, Average Investment in Impaired Loans	29	1	0
Average Investment in Impaired Loans	29	1	0
Real estate - residential mortgage
Financing Receivable, Impaired [Line Items]
Loans without related allowance, Recorded Investment in Impaired Loans	963	1,880	2,195
Loans with related allowance, Recorded Investment in Impaired Loans	781	1,031	832
Recorded Investment in Impaired Loans	1,744	2,911	3,027
Loans without related allowance, Contractual Unpaid Principal Balance	980	3,146	2,242
Loans with related allowance, Contractual Unpaid Principal Balance	919	1,100	865
Contractual Unpaid Principal Balance	1,899	4,246	3,107
Loans with related allowance, Related Allowance	4	31	73
Related Allowance	4	31	73
Loans without related allowance, Interest Income Recognized	15	16	0
Loans with related allowance, Interest Income Recognized	27	30	37
Interest Income Recognized	42	46	37
Loans without related allowance, Average Investment in Impaired Loans	996	2,430	2,096
Loans with related allowance, Average Investment in Impaired Loans	940	1,011	840
Average Investment in Impaired Loans	1,936	3,441	2,936
Real estate - construction
Financing Receivable, Impaired [Line Items]
Loans without related allowance, Recorded Investment in Impaired Loans	1,471	1,471	0
Loans with related allowance, Recorded Investment in Impaired Loans	0	0	380
Recorded Investment in Impaired Loans	1,471	1,471	380
Loans without related allowance, Contractual Unpaid Principal Balance	1,471	1,471	0
Loans with related allowance, Contractual Unpaid Principal Balance	0	0	380
Contractual Unpaid Principal Balance	1,471	1,471	380
Loans with related allowance, Related Allowance	0	0	21
Related Allowance	0	0	21
Loans without related allowance, Interest Income Recognized	0	0	0
Loans with related allowance, Interest Income Recognized	0	0	13
Interest Income Recognized	0	0	13
Loans without related allowance, Average Investment in Impaired Loans	1,471	12	94
Loans with related allowance, Average Investment in Impaired Loans	0	0	308
Average Investment in Impaired Loans	1,471	12	402
Home equity and consumer
Financing Receivable, Impaired [Line Items]
Loans without related allowance, Recorded Investment in Impaired Loans	0	139	574
Loans with related allowance, Recorded Investment in Impaired Loans	993	1,188	1,001
Recorded Investment in Impaired Loans	993	1,327	1,575
Loans without related allowance, Contractual Unpaid Principal Balance	0	139	575
Loans with related allowance, Contractual Unpaid Principal Balance	1,026	1,211	1,013
Contractual Unpaid Principal Balance	1,026	1,350	1,588
Loans with related allowance, Related Allowance	8	94	73
Related Allowance	8	94	73
Loans without related allowance, Interest Income Recognized	0	0	5
Loans with related allowance, Interest Income Recognized	52	59	54
Interest Income Recognized	52	59	59
Loans without related allowance, Average Investment in Impaired Loans	6	388	762
Loans with related allowance, Average Investment in Impaired Loans	1,090	1,184	1,006
Average Investment in Impaired Loans	$ 1,096	$ 1,572	$ 1,768