Off-Balance-Sheet Arrangements, Commitments, Guarantees and Contingencies (Financial Instruments With Off-Balance-Sheet Risk) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Standby Letters of Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	$ 236,595	$ 239,482
Deferred Standby Letter of Credit Fees [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	1,843	2,054
Commitments to Extend Credit [Member]
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Financial instruments with off- balance-sheet risk	$ 7,949,400	$ 7,476,420