Commitments and Contingencies (Outstanding Financial Instruments Contract Amounts Represent Credit Risk) (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Loss Contingencies [Line Items]
Unused Commitments to Extend Credit	$ 6,519,034	$ 6,379,941
Standby letter of credit [Member]
Loss Contingencies [Line Items]
Unused Commitments to Extend Credit	204,981	188,103
Commercial letter of credit [Member]
Loss Contingencies [Line Items]
Unused Commitments to Extend Credit	26,270	29,180
Commitments to extend credit [Member]
Loss Contingencies [Line Items]
Unused Commitments to Extend Credit	$ 6,287,783	$ 6,162,658