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Derivative Financial Instruments - Effect of Derivative Instruments on Consolidated Statement of Earnings (Detail) - Derivatives Not Designated as Hedging Instruments [Member] - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Derivatives, Fair Value [Line Items]      
Amount of Gain Recognized in Income on Derivative Instruments $ 615 $ 691 $ 333
Interest Rate Swaps [Member] | Other Income [Member]      
Derivatives, Fair Value [Line Items]      
Amount of Gain Recognized in Income on Derivative Instruments $ 615 $ 691 $ 333