In May 2017, the Company issued $350 million aggregate principal amount of 0.25% convertible senior notes due June 1, 2022 in a private offering and an additional $50 million aggregate principal amount of such notes pursuant to the exercise in full of the over-allotment options of the initial purchasers (the “2022 Notes”). The 2022 Notes are convertible at the option of the holders prior to the close of business on the business day immediately preceding February 1, 2022 under certain conditions or upon the occurrence of certain events. Upon conversion, the Company will pay or deliver, as the case may be, cash, shares of the Company’s common stock or a combination of cash and shares of the Company’s common stock, at the Company’s election. Because the last reported sale price of the Company’s common stock for at least 20 trading days during the period of 30 consecutive trading days ending on the last trading day of the calendar quarter ended March 31, 2020 was equal to or greater than 130% of the applicable conversion price on each applicable trading day, the 2022 Notes are convertible at the option of the holders thereof during the calendar quarter ending June

30, 2020. In the first quarter of 2020, the Company settled $2 thousand of the principal balance of the 2022 Notes in cash. As of May 1, 2020, the Company has received conversion notices for approximately $2 thousand of the principal balance of the 2022 Notes, which will be settled in cash during the quarter ended June 30, 2020.

The net carrying amount of the liability component of the 2022 Notes is as follows:

	As of March 31, 2020			As of December 31, 2019
	(in thousands)
Principal	$	399,990		$	399,992
Unamortized debt discount		(49,992	)		(55,299	)
Unamortized issuance costs		(3,733	)		(4,129	)
Net carrying amount	$	346,265		$	340,564

	Three Months Ended March 31,
	2020		2019
	(in thousands)
Contractual interest expense	$	250	$	250
Amortization of debt discount		5,307		4,894
Amortization of issuance costs		396		366
Total interest expense	$	5,953	$	5,510