Financial Instruments (Tables)

6 Months Ended

Jun. 30, 2018

Investments All Other Investments [Abstract]

Cash Equivalents, Restricted Cash and Short-Term Investments Classified as Available-for-sale Securities

Cash equivalents, restricted cash and short-term investments which are classified as available-for-sale securities, consisted of the following (in thousands):

June 30, 2018	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Money market funds (cash equivalents)	$	45,949	$	—	$	—		$	45,949
U.S. government agency securities		89,609		5		(1	)		89,613
Restricted cash (money market funds)		175		—		—			175
Total financial assets	$	135,733	$	5	$	(1	)	$	135,737
Classified as:
Cash and cash equivalents								$	45,949
Short-term investments									89,613
Restricted cash (money market funds)									175
								$	135,737

December 31, 2017	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Money market funds (cash equivalents)	$	14,334	$	—	$	—		$	14,334
U.S. government agency securities		16,474		—		(3	)		16,471
Restricted cash (money market funds)		5,330		3		—			5,333
Agency securities		1,500		—		—			1,500
Total financial assets	$	37,638	$	3	$	(3	)	$	37,638
Classified as:
Cash and cash equivalents								$	14,334
Short-term investments									17,971
Restricted cash (money market funds)									5,333
								$	37,638