Loan Aging Analysis by Portfolio Segment (Including Loans Past Due Over Ninety Days and Non Accrual Loans) and Summary of Non Accrual Loans, Which Include Troubled Debt Restructured Loans, and Loans Past Due Over Ninety Days and Accruing (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 days Past Due	$ 3,811	$ 6,613
60-89 days Past Due	1,802	2,470
Greater Than 90 Days	17,375	16,770
Total Past Due	22,988	25,853
Current	1,517,612	1,488,175
Loans ending balance	1,540,600	1,514,028	1,512,312
Loans > 90 Days Past Due and Accruing	246	236
Non-Accrual Loans	22,274	24,110
Residential real estate loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 days Past Due	917	2,207
60-89 days Past Due	755	575
Greater Than 90 Days	7,321	7,373
Total Past Due	8,993	10,155
Current	562,300	568,107
Loans ending balance	571,293	578,262	580,248
Loans > 90 Days Past Due and Accruing		99
Non-Accrual Loans	9,459	9,503
Commercial real estate loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 days Past Due	1,879	2,105
60-89 days Past Due	270	739
Greater Than 90 Days	5,985	5,009
Total Past Due	8,134	7,853
Current	493,149	462,208
Loans ending balance	501,283	470,061	458,348
Loans > 90 Days Past Due and Accruing	209
Non-Accrual Loans	7,121	7,830
Commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 days Past Due	645	1,020
60-89 days Past Due	541	184
Greater Than 90 Days	2,468	2,309
Total Past Due	3,654	3,513
Current	174,629	181,532
Loans ending balance	178,283	185,045	191,060
Loans > 90 Days Past Due and Accruing		135
Non-Accrual Loans	3,765	3,955
Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 days Past Due	359	1,208
60-89 days Past Due	228	962
Greater Than 90 Days	1,341	1,927
Total Past Due	1,928	4,097
Current	272,248	264,685
Loans ending balance	274,176	268,782	270,468
Loans > 90 Days Past Due and Accruing	37	2
Non-Accrual Loans	1,669	2,670
Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 days Past Due	11	73
60-89 days Past Due	8	10
Greater Than 90 Days	260	152
Total Past Due	279	235
Current	15,286	11,643
Loans ending balance	15,565	11,878	12,188
Non-Accrual Loans	$ 260	$ 152