Convertible Senior Notes (Tables)

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Convertible Senior Notes

Convertible senior notes consist of the following (in thousands):

	December 31, 2017						December 31, 2018
	Principal/ Fair Value Amount		Debt Issuance Costs		Net Amount		Principal/ Fair Value Amount		Debt Issuance Costs		Net Amount
4.25% convertible senior notes (due 2018)	$	21,178	$	103	$	21,075	$	—	$	—	$	—
4.875% convertible senior notes (due 2020)	113,000		1,972		111,028		113,000		1,182		111,818
3.25% convertible senior notes (due 2020) *	22,469		—		22,469		27,974		—		27,974
Total convertible senior notes, net of debt issuance costs	$	156,647	$	2,075	$	154,572	$	140,974	$	1,182	$	139,792

* The amount presented for the 3.25% 2020 convertible senior notes within the table represents the fair value. The principal amount of these notes totals $21.5 million and $29.5 million

Components of Convertible Senior Notes

Key components of the 4.25% convertible senior notes due 2018 consist of the following (in thousands):

	Year ended December 31,
	2016		2017		2018
Contractual interest expense	$	4,191	$	2,184	$	373
Amortization of debt issuance costs recognized as interest expense	1,172		844		103
	$	5,363	$	3,028	$	476

Key components of the 4.875% convertible senior notes due 2020 consist of the following (in thousands):

	Year ended December 31,
	2016		2017		2018
Contractual interest expense	$	5,508	$	5,509	$	5,509
Amortization of debt issuance costs recognized as interest expense	804		789		789
	$	6,312	$	6,298	$	6,298

Key components of the 3.25% convertible senior notes due 2020 consist of the following (in thousands):

	Year ended December 31,
	2016		2017		2018
Contractual interest expense	$	—	$	103	$	815
Amortization of debt issuance costs recognized as interest expense	—		—		—
	$	—	$	103	$	815