ACCRUED TRANSACTION LOSSES - Summary of Reserve for Transaction Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss Contingency Accrual [Roll Forward]
Accrued transaction losses, beginning of the year	$ 20,064
Provision for transaction losses	67,018	$ 51,235	$ 54,009
Accrued transaction losses, end of the year	26,893	20,064
Transaction Losses
Loss Contingency Accrual [Roll Forward]
Accrued transaction losses, beginning of the year	20,064	17,176	8,452
Provision for transaction losses	52,977	50,819	43,379
Charge-offs to accrued transaction losses	(46,148)	(47,931)	(34,655)
Accrued transaction losses, end of the year	$ 26,893	$ 20,064	$ 17,176