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Finance Receivables, Credit Quality Information, and Allowance for Credit Losses - Additional Information (Detail) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
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 of the allowance for credit losses was primarily based on historical loss rates. Effective September 30, 2016, it 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)   $ (500)    
Increase (decrease) in net income $ 6,476 $ 6,507 $ 17,564 $ 15,998 $ 23,365
Increase (decrease) in diluted earnings per share $ 0.56 $ 0.50 $ 1.44 $ 1.22  
Impaired finance receivables as a percentage of total finance receivables     1.40%   1.20%
Change in Accounting Method Accounted for as Change in Estimate [Member]          
Financing Receivable, Allowance for Credit Losses [Line Items]          
Increase (decrease) in net income $ 300   $ 300    
Increase (decrease) in diluted earnings per share $ 0.03   $ 0.03