Loan Aging Analysis by Portfolio Segment (Including Loans Past Due Over 90 Days and Non-Accrual Loans) and Summary of Non-Accrual Loans, which Include Troubled Debt Restructured Loans, and Loans Past Due Over 90 Days and Accruing (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 days Past Due	$ 4,333	$ 6,613
60-89 days Past Due	2,794	2,470
Greater Than 90 Days	17,691	16,770
Total Past Due	24,818	25,853
Current	1,511,646	1,488,175
Loans ending balance	1,536,464	1,514,028	1,551,456
Loans > 90 Days Past Due and Accruing	562	236
Non-Accrual Loans	24,310	24,110
Residential real estate loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 days Past Due	842	2,207
60-89 days Past Due	460	575
Greater Than 90 Days	8,206	7,373
Total Past Due	9,508	10,155
Current	560,315	568,107
Loans ending balance	569,823	578,262	590,110
Loans > 90 Days Past Due and Accruing		99
Non-Accrual Loans	10,349	9,503
Commercial real estate loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 days Past Due	2,181	2,105
60-89 days Past Due	1,912	739
Greater Than 90 Days	4,511	5,009
Total Past Due	8,604	7,853
Current	487,807	462,208
Loans ending balance	496,411	470,061	465,248
Loans > 90 Days Past Due and Accruing	451
Non-Accrual Loans	7,362	7,830
Commercial loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 days Past Due	833	1,020
60-89 days Past Due	369	184
Greater Than 90 Days	3,572	2,309
Total Past Due	4,774	3,513
Current	177,654	181,532
Loans ending balance	182,428	185,045	214,415
Loans > 90 Days Past Due and Accruing	111	135
Non-Accrual Loans	4,687	3,955
Home equity loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 days Past Due	425	1,208
60-89 days Past Due	33	962
Greater Than 90 Days	1,142	1,927
Total Past Due	1,600	4,097
Current	270,709	264,685
Loans ending balance	272,309	268,782	269,263
Loans > 90 Days Past Due and Accruing		2
Non-Accrual Loans	1,652	2,670
Consumer loans
Financing Receivable, Recorded Investment, Past Due [Line Items]
30-59 days Past Due	52	73
60-89 days Past Due	20	10
Greater Than 90 Days	260	152
Total Past Due	332	235
Current	15,161	11,643
Loans ending balance	15,493	11,878	12,420
Non-Accrual Loans	$ 260	$ 152