Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 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):

	March 31, 2017
	Level I		Level II		Level III		Total
Financial Assets:
Money market funds	$	309,438	$	—	$	—	$	309,438
Money market funds-restricted	5,497		—		—		5,497
Commercial Paper	29,883		—		—		29,883
U.S. government notes	98,377		—		—		98,377
Corporate bonds	—		168,415		—		168,415
Total financial assets	$	443,195	$	168,415	$	—	$	611,610

	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