Commitments and Contingent Liabilities - Notional Amount of Outstanding Commitments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Commitments to extend credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	$ 1,645,977	$ 1,635,948
Financial standby letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	27,858	36,075
Performance standby letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	26,490	25,915
Commercial letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	$ 2,584	$ 2,611