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Finance Receivables, Credit Quality Information, and Allowance for Credit Losses - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]                          
Increase (decrease) in net income   $ 10,885 $ 5,309 $ 6,135 $ 7,634 $ 6,467 $ 6,476 $ 5,912 $ 5,176 $ 29,963 $ 24,031 $ 23,365  
Increase (decrease) in diluted earnings per share   $ 0.92 $ 0.45 $ 0.52 $ 0.65 $ 0.55 $ 0.56 $ 0.49 $ 0.40 $ 2.54 $ 1.99 $ 1.79  
Increase (decrease) in provision for credit losses   $ 19,464 $ 20,152 $ 18,589 $ 19,134 $ 19,427 $ 16,410 $ 13,386 $ 13,791 $ 77,339 $ 63,014 $ 47,348  
Increase in loss allowance due to change in allowance assumption                   100   500  
Decrease in net income from a change in allowance assumption                   $ (100)   $ (300)  
Decrease in diluted earnings per share from a change in allowance assumption                   $ 0.01   $ (0.02)  
Allowance for credit losses as percentage of loans   6.00%       5.70%       6.00% 5.70% 6.00%  
Impaired finance receivables as a percentage of total finance receivables                   2.10% 1.60%    
Previous [Member]                          
Financing Receivable, Allowance for Credit Losses [Line Items]                          
Loss emergence period of large loan finance receivable                   12 months   10 months  
Loss emergence period of small loan finance receivable                   6 months      
Loss emergence period of retail loan finance receivable                   10 months      
New [Member]                          
Financing Receivable, Allowance for Credit Losses [Line Items]                          
Loss emergence period of large loan finance receivable                   10 months   12 months  
Loss emergence period of small loan finance receivable                   7 months      
Loss emergence period of retail loan finance receivable                   11 months      
Large Loans [Member]                          
Financing Receivable, Allowance for Credit Losses [Line Items]                          
Allowance for credit losses as percentage of loans   5.10%       4.90%       5.10% 4.90% 3.80% 4.30%
Hurricanes [Member]                          
Financing Receivable, Allowance for Credit Losses [Line Items]                          
Increase (decrease) in net income     $ (1,900)                    
Increase (decrease) in diluted earnings per share     $ (0.16)                    
Increase (decrease) in provision for credit losses $ 3,000                        
Bulk Sale of Charged Off Loans [Member]                          
Financing Receivable, Allowance for Credit Losses [Line Items]                          
Recoveries $ 1,000   $ 1,000                 $ 2,000  
Change in Accounting Method Accounted for as Change in Estimate [Member]                          
Financing Receivable, Allowance for Credit Losses [Line Items]                          
Allowance for credit losses - factors influencing management's judgment                   Prior to September 30, 2016, the general component reflected estimated credit losses for groups of finance receivables on a collective basis and was primarily based on historical loss rates (adjusted for qualitative factors). Effective September 30, 2016, the general component is primarily based on delinquency roll rates. Delinquency roll rate modeling is forward-looking and common practice in the consumer finance industry.      
Increase (decrease) in provision for credit losses                     $ (500)    
Increase (decrease) in net income                     $ 300    
Increase (decrease) in diluted earnings per share                     $ 0.03