4. Financial Instruments

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

	September 30, 2018									December 31, 2017
	Cost		Unrealized Gain		Unrealized (Loss)			Estimated Fair Value		Cost			Unrealized Gain		Unrealized (Loss)			Estimated Fair Value
Money market funds	$	15,160	$	—	$	—		$	15,160		$	12,430	$	—	$	—		$	12,430
Corporate notes and commercial paper		78,279		—		(38	)		78,241			85,553		—		(61	)		85,492
U.S. treasury securities		13,949		—		(13	)		13,936			34,505		—		(68	)		34,437
U.S. government agency securities		33,423		—		(122	)		33,301			53,832		—		(141	)		53,691
	$	140,811	$	—	$	(173	)	$	140,638		$	186,320	$	—	$	(270	)	$	186,050
Classified as:
Cash equivalents								$	59,877									$	47,931
Short-term investments									80,321										115,318
Long-term investments									—										22,361
Restricted cash									440										440
Total cash equivalents, restricted cash and investments								$	140,638									$	186,050

At September 30, 2018, the remaining contractual maturities of available-for-sale securities were less than one year. There have been no significant realized gains or losses on available-for-sale securities for the periods presented. As of September 30, 2018, unrealized losses on cash equivalents and investments were $173,000 and the losses were deemed to be temporary. The gross unrealized loss that had been in a continuous loss position for 12 months or longer was $0.1 million as of September 30, 2018. There were no investments in a continuous loss position for 12 months or longer as of December 31, 2017. The Company does not intend to sell its securities that are in an unrealized loss position, and it is unlikely that the Company will be required to sell its securities before recovery of their amortized cost basis, which may be maturity. Factors considered in determining whether a loss is temporary include the length of time and extent to which the fair value has been less than the amortized cost basis and whether the Company intends to sell the security or whether it is more likely than not that the Company would be required to sell the security before recovery of the amortized cost basis. As of September 30, 2018, the Company had a total of $141.9 million in cash, cash equivalents, restricted cash and investments, which includes $1.3 million in cash and $140.6 million in cash equivalents, restricted cash and investments.