Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value, Assets Measured on Recurring and Nonrecurring Basis

December 31, 2018	Fair Value Measurements
(in thousands)	Total		Level 1		Level 2		Level 3
Money market mutual funds	$	47,199	$	47,199	$	—	$	—
Total cash equivalents	$	47,199	$	47,199	$	—	$	—
U.S. government and agency bonds	$	112,509	$	69,605	$	42,904	$	—
Corporate bonds	56,025		—		56,025		—
Certificates of deposit	6,506		—		6,506		—
Total short-term investments	$	175,040	$	69,605	$	105,435	$	—
U.S. government and agency bonds	$	90,402	$	64,086	$	26,316	$	—
Corporate bonds	46,718		—		46,718		—
Certificates of deposit	3,664		—		3,664		—
Total long-term investments	$	140,784	$	64,086	$	76,698	$	—

Fair Value, Liabilities Measured on Recurring and Nonrecurring Basis

The carrying amount and the estimated fair value of the Company's convertible debt, which is based on the Level 2 quoted market prices as of December 31, 2018 and 2017 are as follows:

	As of
	December 31, 2018				December 31, 2017
(in thousands)	Carrying Value		Estimated Fair Value		Carrying Value		Estimated Fair Value
2% Convertible Senior Notes	$	—	$	—	$	3,421	$	5,467
1.375% Convertible Senior Notes	290,972		426,026		276,172		407,652
1.25% Convertible Senior Notes	301,006		483,851		286,580		450,881
Total	$	591,978	$	909,877	$	566,173	$	864,000