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Finance Receivables, Credit Quality Information, and Allowance for Credit Losses - Additional Information (Detail) - USD ($)
$ in Thousands
1 Months Ended 3 Months Ended 6 Months Ended 12 Months Ended
Sep. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]            
Increase (decrease) in provision for credit losses   $ 20,203 $ 18,589 $ 39,718 $ 37,723  
Impaired finance receivables as a percentage of total finance receivables       2.40%   2.10%
Hurricanes [Member]            
Financing Receivable, Allowance for Credit Losses [Line Items]            
Increase (decrease) in provision for credit losses $ 3,000