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Securities Available for Sale - Securities Available for Sale by Contractual Maturity (Detail) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Amortized cost $ 1,042,285 $ 1,041,247
Securities available for sale, at fair value 1,037,812 $ 1,040,386
Available-for-sale Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Due in one year or less, amortized cost 20,406  
Due after one year through five years, amortized cost 3,009  
Due after five years through ten years, amortized cost 24,905  
Amortized cost 48,320  
Due in one year or less, fair value 20,390  
Due after one year through five years, fair value 3,048  
Due after five years through ten years, fair value 25,349  
Securities available for sale, at fair value $ 48,787