Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2015

Fair Value Disclosures [Abstract]

Fair value of financial assets by level

We measure and report our cash equivalents and available-for-sale marketable securities at fair value. The following table sets forth the fair value of our financial assets by level within the fair value hierarchy (in thousands):

	March 31, 2015
	Level I		Level II		Level III		Total
Financial Assets
Cash and cash equivalents:
Money market funds	$	104,305	$	—	$	—	$	104,305
Marketable securities:
U.S. government notes	209,153		—		—		209,153
Total financial assets	$	313,458	$	—	$	—	$	313,458

	December 31, 2014
	Level I		Level II		Level III		Total
Financial Assets
Cash and cash equivalents:
Money market funds	$	104,216	$	—	$	—	$	104,216
Marketable securities:
U.S. government notes	209,426		—		—		209,426
Total financial assets	$	313,642	$	—	$	—	$	313,642