Commitments and Contingent Liabilities - Notional Amount of Outstanding Commitments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Commitments to extend credit [Member]
Contingencies And Commitments [Line Items]
Financial instrument of credit risk	$ 1,515,221	$ 1,495,009
Financial standby letters of credit [Member]
Contingencies And Commitments [Line Items]
Financial instrument of credit risk	48,864	53,689
Performance standby letters of credit [Member]
Contingencies And Commitments [Line Items]
Financial instrument of credit risk	77,795	76,371
Commercial letters of credit [Member]
Contingencies And Commitments [Line Items]
Financial instrument of credit risk	$ 1,047	$ 1,297