Commitments and Contingencies - Outstanding Financial Instruments Contract Amounts Represent Credit Risk (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Loss Contingencies [Line Items]
Total credit-related financial instruments with off-balance sheet risk	$ 5,062,933	$ 4,547,484
Standby letters of credit [Member]
Loss Contingencies [Line Items]
Total credit-related financial instruments with off-balance sheet risk	146,932	142,964
Commercial letters of credit [Member]
Loss Contingencies [Line Items]
Total credit-related financial instruments with off-balance sheet risk	32,779	27,787
Commitments to extend credit [Member]
Loss Contingencies [Line Items]
Total credit-related financial instruments with off-balance sheet risk	$ 4,883,222	$ 4,376,733