Fair Value Measurements (Tables)

6 Months Ended

Jun. 30, 2018

Fair Value Measurements

Schedule of assets measured at fair value on a recurring basis

Assets measured at fair value on a recurring basis are summarized below (in thousands):

	Fair Value Measurements as of June 30, 2018
	Total		Level 1		Level 2		Level 3
Marketable Securities
Bonds and bond funds	$	206,184	$	—	$	206,184	$	—
Commercial paper		267,178		—		267,178		—
	$	473,362	$	—	$	473,362	$	—
Cash, Cash Equivalents		175,728		175,728		—		—
Total Assets Measured at Fair Value	$	649,090	$	175,728	$	473,362	$	—

	Fair Value Measurements as of December 31, 2017
	Total		Level 1		Level 2		Level 3
Marketable Securities
Bonds and bond funds	$	344,251	$	—	$	344,251	$	—
Commercial paper		159,252		—		159,252		—
	$	503,503	$	—	$	503,503	$	—
Cash, Cash Equivalents		181,792		181,792		—		—
Total Assets Measured at Fair Value	$	685,295	$	181,792	$	503,503	$	—