Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Fair Value of Financial Assets by Level

The following table set forth the fair value of our financial assets by level within the fair value hierarchy (in thousands):

	December 31, 2017
	Level I		Level II		Level III		Total
Financial Assets:
Money market funds	$	701,145	$	—	$	—	$	701,145
Money market funds - restricted	5,505		—		—		5,505
Commercial Paper	—		11,924		—		11,924
U.S. government notes	136,647		—		—		136,647
Corporate bonds	—		312,484		—		312,484
Agency securities	—		228,036		—		228,036
Total financial assets	$	843,297	$	552,444	$	—	$	1,395,741

	December 31, 2016
	Level I		Level II		Level III		Total
Financial Assets:
Money market funds	$	305,182	$	—	$	—	$	305,182
Money market funds - restricted	4,245		—		—		4,245
Commercial Paper	—		5,962		—		5,962
U.S. government notes	110,756		—		—		110,756
Corporate bonds	—		183,192		—		183,192
Total financial assets	$	420,183	$	189,154	$	—	$	609,337