Commitments and Contingent Liabilities - Notional Amount of Outstanding Commitments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Commitments to extend credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	$ 1,474,447	$ 1,506,618
Financial standby letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	40,829	47,185
Performance standby letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	49,249	69,240
Commercial letters of credit [Member]
Loss Contingencies [Line Items]
Financial instrument of credit risk	$ 330	$ 685