Convertible Notes (Tables)

12 Months Ended

Dec. 31, 2017

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, 2017			As of December 31, 2016
1.375% Convertible Senior Notes due 2023	$	172,500		$	—
1.125% Convertible Senior Notes due 2018	81,207			138,000
Total principal amount of convertible notes	253,707			138,000
Unamortized discount - 2023 Notes	(34,506		)	—
Unamortized discount - 2018 Notes	(2,547		)	(10,913		)
Unamortized debt issuance costs - 2023 Notes	(2,547		)	—
Unamortized debt issuance costs - 2018 Notes	(209		)	(920		)
Total convertible notes	$	213,898		$	126,167
Less current portion	78,451			—
Total long-term convertible notes	$	135,447		$	126,167

Schedule of interest expense on notes

Interest expense related to the notes for the years ended December 31, 2017, 2016 and 2015 was as follows:

	Years Ended December 31,
	2017		2016		2015
	(in thousands)
2023 Notes coupon interest at a rate of 1.375%	$	290	$	—	$	—
2023 Notes amortization of discount and debt issuance cost at an additional effective interest rate of 4.9%	768		—		—
2018 Notes coupon interest at a rate of 1.125%	1,488		1,553		1,567
2018 Notes amortization of discount and debt issuance cost at an additional effective interest rate of 5.5%	6,810		6,749		6,372
Total interest expense on convertible notes	$	9,356	$	8,302	$	7,939