Fair Value Measurements (Tables)

9 Months Ended

Sep. 30, 2017

Fair Value Disclosures [Abstract]

Fair value of financial assets by level

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

	September 30, 2017
	Level I		Level II		Level III		Total
Financial Assets:
Money market funds	$	319,584	$	—	$	—	$	319,584
Money market funds-restricted	5,502		—		—		5,502
Commercial paper	11,879		—		—		11,879
U.S. government notes	104,722		—		—		104,722
Corporate bonds	—		372,034		—		372,034
Total financial assets	$	441,687	$	372,034	$	—	$	813,721

	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	$	426,145	$	183,192	$	—	$	609,337