Loans and Allowance for Credit Losses - Schedule of Troubled Debt Restructurings for which there was Payment Default (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2013 SecurityLoan	Dec. 31, 2012 Troubled Debt Restructurings that Subsequently Defaulted [Member] SecurityLoan	Sep. 30, 2013 Commercial Loan [Member] SecurityLoan	Dec. 31, 2012 Commercial Loan [Member] Troubled Debt Restructurings that Subsequently Defaulted [Member] SecurityLoan	Sep. 30, 2013 Commercial Real Estate [Member] SecurityLoan	Dec. 31, 2012 Commercial Real Estate [Member] Troubled Debt Restructurings that Subsequently Defaulted [Member] SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Contracts	3	15	1	8	2	7
Recorded Investment	$ 391	$ 1,111	$ 76	$ 500	$ 315	$ 611