Loans and Allowance for Credit Losses - Troubled Debt Restructuring is 90 Days or More Past Due (Detail) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) Contract	Sep. 30, 2014 USD ($) Contract	Sep. 30, 2015 USD ($) Contract	Sep. 30, 2014 USD ($) Contract
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Modifications, Subsequent Default, Number of Contracts			3	3
Financing Receivable, Modifications, Subsequent Default, Recorded Investment | $			$ 108	$ 18
Troubled Debt Restructuring Number Of Contracts Ninety Days Past Due	2	1
Financing Receivable, Modification, Subsequent Default, Recorded Investment | $	$ 105	$ 5
Residential real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Modifications, Subsequent Default, Number of Contracts			3	3
Financing Receivable, Modifications, Subsequent Default, Recorded Investment | $			$ 108	$ 18
Troubled Debt Restructuring Number Of Contracts Ninety Days Past Due	2	1
Financing Receivable, Modification, Subsequent Default, Recorded Investment | $	$ 105	$ 5