DEBT (Tables)

6 Months Ended

Jun. 30, 2019

Long-term Debt, Unclassified [Abstract]

Schedule of principal amount of the Senior Notes is repayable

The principal amount of the Senior Notes is repayable as of June 30, 2019 is as follows (in thousands):

2019 (remainder)	$	40,000
2020	80,000
2021	82,500
Total	$	202,500

Summary of the carrying value of the Senior Notes

The following is a summary of the carrying value of the Senior Notes as of June 30, 2019 and December 31, 2018 (in thousands): 

	June 30, 2019			December 31, 2018
Principal amount of the Senior Notes	$	202,500		$	282,500
Unamortized debt discount balance	(1,698		)	(2,541		)
Unamortized debt issuance costs	(3,275		)	(1,650		)
Total Senior Notes	$	197,527		$	278,309

Summary of interest expense for the Notes

The following is a summary of interest expense for the three and six months ended June 30, 2019 and 2018 (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Stated coupon interest	$	2,156	$	2,156	$	4,312	$	4,312
Amortization of debt discount and debt issuance costs	4,945		4,537		9,752		8,947
Total interest expense Convertible Notes	$	7,101	$	6,693	$	14,064	$	13,259

The following is a summary of interest expense for the three and six months ended June 30, 2019 and 2018 (in thousands): 

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Contractual interest expense	$	6,602	$	9,425	$	14,808	$	19,866
Amortization of debt discount and debt issuance costs	1,111		853		2,468		1,861
Total interest expense Senior Notes	$	7,713	$	10,278	$	17,276	$	21,727

Summary of the liability component of the Convertible Notes

The following is a summary of the liability component of the Convertible Notes as of June 30, 2019 and December 31, 2018 (in thousands):

	June 30, 2019			December 31, 2018
Principal amount of the Convertible Notes	$	345,000		$	345,000
Unamortized discount of the liability component	(45,273		)	(54,521		)
Unamortized debt issuance costs	(2,177		)	(2,681		)
Total Convertible Notes	$	297,550		$	287,798