DEBT (Tables)

9 Months Ended

Sep. 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 September 30, 2019 is as follows (in thousands):

2019 (remainder)	$	20,000
2020	80,000
2021	82,500
Total	$	182,500

Summary of the carrying value of the Senior Notes

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

	September 30, 2019			December 31, 2018
Principal amount of the Senior Notes	$	182,500		$	282,500
Unamortized debt discount balance	(5,220		)	(2,541		)
Unamortized debt issuance costs	(2,618		)	(1,650		)
Total Senior Notes	$	174,661		$	278,309

Summary of interest expense for the Notes

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

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Stated coupon interest	$	1,490	$	2,156	$	5,802	$	6,468
Amortization of debt discount and debt issuance costs	3,499		4,604		13,252		13,551
Total interest expense 2021 Notes	$	4,989	$	6,760	$	19,054	$	20,019

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

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Contractual interest expense	$	5,736	$	9,517	$	20,544	$	29,383
Amortization of debt discount and debt issuance costs	1,534		886		4,001		2,747
Total interest expense Senior Notes	$	7,270	$	10,403	$	24,545	$	32,130

The following is a summary of interest expense for the 2024 Notes for three and nine months ended September 30, 2019 (in thousands):

	Three and Nine Months Ended September 30,
	2019
Stated coupon interest	$	750
Amortization of debt discount and debt issuance costs	837
Total interest expense 2024 Notes	$	1,587

Summary of the liability component of the Convertible Notes

The following is a summary of the liability component of the 2021 Notes as of September 30, 2019 and December 31, 2018 (in thousands):

	September 30, 2019			December 31, 2018
Principal amount of the 2021 Notes	$	145,000		$	345,000
Unamortized discount of the liability component	(17,016		)	(54,521		)
Unamortized debt issuance costs	(817		)	(2,681		)
Total 2021 Notes	$	127,166		$	287,798

The 2024 Notes are not convertible or redeemable as of September 30, 2019. The following is a summary of the liability component of the 2024 Notes as of September 30, 2019 (in thousands):

	September 30, 2019
Principal amount of the 2024 Notes	$	120,000
Unamortized discount of the liability component	(53,407		)
Unamortized debt issuance costs	(2,836		)
Total 2024 Notes	$	63,757