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 September 30, 2019 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 December 31, 2019. As of November 1, 2019, the Company has received conversion notices for approximately $8 thousand of the principal balance of the 2022 Notes, which will be settled in cash during the quarter ended December 31, 2019.

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

	As of September 30, 2019			As of December 31, 2018
	(in thousands)
Principal	$	400,000		$	400,000
Unamortized debt discount		(60,516	)		(75,575	)
Unamortized issuance costs		(4,518	)		(5,643	)
Net carrying amount	$	334,966		$	318,782

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
	(in thousands)				(in thousands)
Contractual interest expense	$	250	$	250	$	750	$	750
Amortization of debt discount		5,126		4,784		15,060		14,054
Amortization of issuance costs		383		357		1,124		1,049
Total interest expense	$	5,759	$	5,391	$	16,934	$	15,853