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Finance Receivables, Credit Quality Information, and Allowance for Credit Losses - Summary of Changes in Allowance for Credit Losses (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Receivables [Abstract]      
Balance at beginning of period $ 41,250 $ 37,452 $ 40,511
Provision for credit losses 77,339 63,014 47,348
Credit losses (75,880) (64,064) (55,043)
Recoveries 6,201 4,848 4,636
Balance at end of period $ 48,910 $ 41,250 $ 37,452