Cash and Cash Equivalents and Investments (Tables)

12 Months Ended

Dec. 31, 2017

Cash and Cash Equivalents and Investments [Abstract]

Schedule of Cash, Cash Equivalents and Short-term Investments

	As of December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
			(in thousands)
Cash and cash equivalents	$	61,661	$	—	$	—		$	61,661
Securities maturing within one year:
U.S. treasury securities	222,316		—		(168		)	222,148
Corporate debt securities	27,033		—		(54		)	26,979
Total short-term investments	$	249,349	$	—	$	(222	)	$	249,127
Total cash and cash equivalents and investments	$	311,010	$	—	$	(222	)	$	310,788

	As of December 31, 2016
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
			(in thousands)
Cash and cash equivalents	$	46,600	$	—	$	—		$	46,600
Securities maturing within one year:
U.S. treasury securities	227,911		1		(107		)	227,805
Corporate debt securities	72,188		1		(90		)	72,099
Total short-term investments	$	300,099	$	2	$	(197	)	$	299,904
Total cash and cash equivalents and investments	$	346,699	$	2	$	(197	)	$	346,504