Loans - Loans by Class Modified as Troubled Debt Restructurings (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended	12 Months Ended	9 Months Ended			12 Months Ended	9 Months Ended			12 Months Ended	9 Months Ended			12 Months Ended	9 Months Ended			12 Months Ended	9 Months Ended			12 Months Ended	9 Months Ended
	Sep. 30, 2013 Loans	Sep. 30, 2012 Loans	Sep. 30, 2013 Loans	Dec. 31, 2012 Loans	Sep. 30, 2013 Trouble Debt Restructuring [Member] Loans	Sep. 30, 2012 Trouble Debt Restructuring [Member] Loans	Sep. 30, 2013 Commercial, Industrial, and Agricultural [Member] Loans	Dec. 31, 2012 Commercial, Industrial, and Agricultural [Member] Loans	Sep. 30, 2013 Commercial, Industrial, and Agricultural [Member] Trouble Debt Restructuring [Member] Loans	Sep. 30, 2012 Commercial, Industrial, and Agricultural [Member] Trouble Debt Restructuring [Member] Loans	Sep. 30, 2013 Commercial Mortgages [Member] Loans	Dec. 31, 2012 Commercial Mortgages [Member] Loans	Sep. 30, 2013 Commercial Mortgages [Member] Trouble Debt Restructuring [Member] Loans	Sep. 30, 2012 Commercial Mortgages [Member] Trouble Debt Restructuring [Member] Loans	Sep. 30, 2013 Residential Real Estate [Member] Loans	Dec. 31, 2012 Residential Real Estate [Member] Loans	Sep. 30, 2013 Residential Real Estate [Member] Trouble Debt Restructuring [Member] Loans	Sep. 30, 2012 Residential Real Estate [Member] Trouble Debt Restructuring [Member] Loans	Sep. 30, 2013 Consumer [Member] Loans	Dec. 31, 2012 Consumer [Member] Loans	Sep. 30, 2013 Consumer [Member] Trouble Debt Restructuring [Member] Loans	Sep. 30, 2012 Consumer [Member] Trouble Debt Restructuring [Member] Loans	Sep. 30, 2013 Credit Cards [Member] Loans	Dec. 31, 2012 Credit Cards [Member] Loans	Sep. 30, 2013 Credit Cards [Member] Trouble Debt Restructuring [Member] Loans	Sep. 30, 2012 Credit Cards [Member] Trouble Debt Restructuring [Member] Loans
Financing Receivable, Modifications [Line Items]
Number of Loans	1	0	12	10	2	5	2	2	0	1	10	8	2	4	0	0	0	0	0	0	0	0	0	0	0	0
Pre-Modification Outstanding Recorded Investment					$ 3,615	$ 2,866			$ 0	$ 310			$ 3,615	$ 2,556			$ 0	$ 0			$ 0	$ 0			$ 0	$ 0
Post-Modification Outstanding Recorded Investment					$ 3,549	$ 2,866			$ 0	$ 310			$ 3,549	$ 2,556			$ 0	$ 0			$ 0	$ 0			$ 0	$ 0