Investments (Tables)

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Schedule of marketable securities

The following is a summary of investments in marketable securities, including those that meet the definition of a cash equivalent, as of December 31, 2017 (in thousands):

	December 31, 2017
	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value		Cash Equivalent		Investments
Money market funds	$	358,859	$	—	$	—		$	358,859	$	358,859	$	—
Certificate of deposit	10,000		—		—			10,000		10,000		—
Corporate bonds	74,868		—		(220		)	74,648		—		74,648
U.S. treasury securities	189,310		—		(430		)	188,880		—		188,880
	$	633,037	$	—	$	(650	)	$	632,387	$	368,859	$	263,528

The following is a summary of investments in marketable securities, including those that meet the definition of a cash equivalent, as of December 31, 2016 (in thousands):

	December 31, 2016
	Amortized Cost Basis		Unrealized Gains		Unrealized Losses			Fair Value		Cash Equivalent		Investments
Money market funds	$	48,136	$	—	$	—		$	48,136	$	48,136	$	—
Corporate bonds	60,725		1		(50		)	60,676		—		60,676
Agency bonds	28,954		2		(26		)	28,930		—		28,930
U.S. treasury securities	157,829		17		(160		)	157,686		—		157,686
Commercial paper	10,473		—		—			10,473		—		10,473
	$	306,117	$	20	$	(236	)	$	305,901	$	48,136	$	257,765