Commitments and Letters of Credit - Notional Amount of Commitments and Letter of Credit Instruments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Commitments to extend credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	$ 1,635,948	$ 1,571,987
Financial standby letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	36,075	38,121
Performance standby letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	25,915	32,441
Commercial letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	$ 2,611	$ 0