Retail Installment Contract Receivables - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Deferred revenue	$ 91,719	$ 114,345
Retail Installment Contracts
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Deferred interest, beginning of period	34,163	$ 36,048	$ 36,048
Write-offs, net of recoveries	(12,500)		(26,360)
Deferred revenue	9,989		24,475
Deferred interest, end of period	$ 31,652		$ 34,163