Commitments and Contingencies (Outstanding Financial Instruments Contract Amounts Represent Credit Risk) (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Total credit-related financial instruments with off-balance sheet risk	$ 5,391,027	$ 5,399,762
Standby letter of credit [Member]
Loss Contingencies [Line Items]
Total credit-related financial instruments with off-balance sheet risk	138,536	128,985
Commercial letter of credit [Member]
Loss Contingencies [Line Items]
Total credit-related financial instruments with off-balance sheet risk	40,678	46,497
Commitments to extend credit [Member]
Loss Contingencies [Line Items]
Total credit-related financial instruments with off-balance sheet risk	$ 5,211,813	$ 5,224,280