DEBT (Tables)

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Schedule of principal amounts due

The remaining principal amount of the Senior Notes repayable each year is as follows (in thousands):

2019	$	120,000
2020	80,000
2021	82,500
Total	$	282,500

Summary of the carrying values

The following is a summary of the carrying value of the Senior Notes as of December 31, 2018 and 2017 (in thousands):

	December 31,			December 31,
	2018			2017
Principal amount of the Senior Notes	$	282,500		$	365,000
Unamortized debt discount balance	(2,541		)	(4,717		)
Unamortized debt issuance costs	(1,650		)	(3,063		)
Total Senior Notes	$	278,309		$	357,220

Summary of the liability components

The following is a summary of the liability component of the Convertible Notes as of December 31, 2018 and 2017 (in thousands):

	December 31,			December 31,
	2018			2017
Principal amount of the Convertible Notes	$	345,000		$	345,000
Unamortized discount of the liability component	(54,521		)	(71,799		)
Unamortized debt issuance costs	(2,681		)	(3,691		)
Total Convertible Notes	$	287,798		$	269,510

Summary of interest expense

The following is a summary of interest expense for 2018, 2017 and 2016 (in thousands):

	December 31,
	2018		2017		2016
Stated coupon interest	$	8,624	$	8,625	$	8,625
Amortization of debt discount and debt issuance costs	18,288		16,784		15,412
Total interest expense	$	26,912	$	25,409	$	24,037

The debt discount and debt issuance costs will be amortized as interest expense through April 2021. The following is a summary of Senior Notes interest expense (in thousands):

	December 31,
	2018		2017		2016
Contractual interest expense	$	38,242	$	44,212	$	54,722
Amortization of debt discount and debt issuance costs	3,589		2,631		2,261
Total interest expense	$	41,831	$	46,843	$	56,983