Loans and Allowance for Credit Losses - Troubled Debt Restructured Loans and Commitments (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financing Receivable, Troubled Debt Restructurings [Line Items]
Financing Receivable, Modifications, Recorded Investment	$ 20,607	$ 29,536
Unused lines of Credit [Member]
Financing Receivable, Troubled Debt Restructurings [Line Items]
Loans and Leases Receivable, Impaired, Commitment to Lend	1,162	4,120
Accrual Loans [Member] [Member]
Financing Receivable, Troubled Debt Restructurings [Line Items]
Financing Receivable, Modifications, Recorded Investment	12,024	12,584
Non Accrual Loans [Member]
Financing Receivable, Troubled Debt Restructurings [Line Items]
Financing Receivable, Modifications, Recorded Investment	$ 8,583	$ 16,952