Convertible Notes (Tables)

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Schedule of convertible notes

The Company’s convertible notes are shown in the following table.

(Dollars in thousands)	As of December 31, 2018			As of December 31, 2017
1.375% Convertible Senior Notes due 2023	$	172,500		$	172,500
1.125% Convertible Senior Notes due 2018	—			81,207
Total principal amount of convertible notes	172,500			253,707
Unamortized discount - 2023 Notes	(28,517		)	(34,506		)
Unamortized discount - 2018 Notes	—			(2,547		)
Unamortized debt issuance costs - 2023 Notes	(2,049		)	(2,547		)
Unamortized debt issuance costs - 2018 Notes	—			(209		)
Total convertible notes	$	141,934		$	213,898
Less current portion	—			78,451
Total long-term convertible notes	$	141,934		$	135,447

Schedule of interest expense on notes

Interest expense related to the notes for the years ended December 31, 2018, 2017 and 2016 was as follows:

	Years Ended December 31,
	2018		2017		2016
	(in thousands)
2023 Notes coupon interest at a rate of 1.375%	$	2,372	$	290	$	—
2023 Notes amortization of discount and debt issuance cost at an additional effective interest rate of 4.9%	6,486		768		—
2018 Notes coupon interest at a rate of 1.125%	377		1,488		1,553
2018 Notes amortization of discount and debt issuance cost at an additional effective interest rate of 5.5%	2,756		6,810		6,749
Total interest expense on convertible notes	$	11,991	$	9,356	$	8,302