Derivative Financial Instruments - Effect of Derivative Instruments on Consolidated Statements of Income Derivative in Fair Value Hedging Relationship (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Interest Income - Loans [Member] | Derivative in Fair Value Hedging Relationship [Member] | Interest Rate Contracts One [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized on Derivative	$ 2,768	$ 253
Interest Income - Loans [Member] | Derivative in Fair Value Hedging Relationship [Member] | Interest Rate Contracts Two [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized on Derivative	(2,768)	(253)
Derivatives Not Designated as Hedging Instruments [Member] | Other income - Gain on Sale of Loans [Member] | Mortgage Loan Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized on Derivative	$ 112	$ (59)