Loans and Allowance for Credit Losses - Troubled Debt Restructuring is 90 Days or More Past Due (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012 Contract	Sep. 30, 2011 Contract
Financing Receivable, Recorded Investment, Past Due [Line Items]
Troubled debt restructuring number of contracts 90 days past due	0	1
Troubled debt restructuring recorded investment 90 days past due	$ 0	$ 88
Real estate construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Troubled debt restructuring number of contracts 90 days past due	0	1
Troubled debt restructuring recorded investment 90 days past due	$ 0	$ 88