Debt (Tables)

12 Months Ended

Dec. 31, 2018

Debt

Schedule of carrying value of Convertible Senior Notes

	December 31,
	2018		2017
	(in thousands)
Principal amount	$	345,000	$	345,000
Unamortized debt discount		(52,336)		(63,022)
Unamortized transaction costs		(5,272)		(6,348)
Net carrying value	$	287,392	$	275,630

Schedule of interest expense related to Convertible Senior Notes

	For the year ended December 31,
	2018		2017
	(in thousands)
Cash Interest Expense
Coupon interest expense	$	9,315	$	8,901
Non-Cash Interest Expense
Amortization of debt discount		10,686		9,490
Amortization of transaction costs		1,076		956
Total Interest Expense	$	21,077	$	19,347