Commitments and Contingencies (Outstanding Financial Instruments Contract Amounts Represent Credit Risk) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Total credit-related financial instruments with off-balance sheet risk	$ 5,806,789	$ 5,399,762
Standby letters of credit [Member]
Loss Contingencies [Line Items]
Total credit-related financial instruments with off-balance sheet risk	195,902	128,985
Letter of Credit [Member]
Loss Contingencies [Line Items]
Total credit-related financial instruments with off-balance sheet risk	43,200	46,497
Commitments to extend credit [Member]
Loss Contingencies [Line Items]
Total credit-related financial instruments with off-balance sheet risk	$ 5,567,687	$ 5,224,280