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Quarterly Information (Unaudited) - Summary of the Company's Quarterly Financial Information (Parenthetical) (Detail) - USD ($)
$ in Thousands
1 Months Ended 3 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2017
Sep. 30, 2017
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2015
Quarterly Information [Line Items]            
Tax benefit related to the exercise of stock options       $ 1,500    
Incremental hurricane allowance for credit losses     $ 3,000      
Provisional benefit to income tax expense, Tax Act   $ 3,100     $ 3,122  
Bulk Sale of Charged Off Loans [Member]            
Quarterly Information [Line Items]            
Recovery from bulk sale of previously charged-off customer accounts in bankruptcy $ 1,000   $ 1,000     $ 2,000