Cash, Cash Equivalents, Marketable Securities and Restricted Cash

As of March 31, 2019, the Company held $96.3 million in cash and cash equivalents and $342.0 million of available-for-sale debt securities which are reported at fair value on the Company's consolidated balance sheets. Unrealized holding gains and losses are reported within accumulated other comprehensive loss in the statements of comprehensive loss. If a decline in the fair value of a marketable security below the Company's cost basis is determined to be other-than-temporary, such marketable security is written down to its estimated fair value as a new cost basis and the amount of the write-down is included in earnings as an impairment charge.

The Company regularly invests excess operating cash in deposits with major financial institutions, money market funds, notes issued by the U.S. government, as well as fixed income investments and U.S. bond funds, both of which can be readily purchased and sold using established markets. The Company believes that the market risk arising from its holdings of these financial instruments is mitigated as many of these securities are either government backed or of the highest credit rating. Investments that have original maturities greater than three months but less than one year are classified as current, while investments that have maturities greater than one year are classified as long-term.

Cash, cash equivalents and marketable securities are classified as current unless mentioned otherwise below and consisted of the following:



	As of March 31, 2019
(in thousands)	Cost		Gross unrealized Gain		Gross unrealized Loss			Fair Value
Cash and cash equivalents	$	96,349	$	—	$	—		$	96,349
Corporate debt securities, current portion	138,194		96		(6		)	138,284
Commercial paper	134,948		52		(7		)	134,993
Asset-backed securities	68,278		30		(8		)	68,300
Money market	350		—		—			350
Certificates of deposit	51		—		—			51
	$	438,170	$	178	$	(21	)	$	438,327
Included in cash and cash equivalents	$	96,349	$	—	$	—		$	96,349
Included in marketable securities, current and non-current	341,821		178		(21		)	341,978
Total cash, cash equivalents and marketable securities	$	438,170	$	178	$	(21	)	$	438,327



	As of December 31, 2018
(in thousands)	Cost		Gross unrealized Gain		Gross unrealized Loss			Fair Value
Cash and cash equivalents	$	79,749	$	—	$	—		$	79,749
Corporate debt securities, current portion	240,969		7		(250		)	240,726
Commercial paper	115,245		—		(104		)	115,141
Asset-backed securities	68,215		4		(84		)	68,135
Money market	350		—		—			350
Certificates of deposit	51		—		—			51
	$	504,579	$	11	$	(438	)	$	504,152
Included in cash and cash equivalents	$	79,749	$	—	$	—		$	79,749
Included in marketable securities	424,830		11		(438		)	424,403
Total cash, cash equivalents and marketable securities	$	504,579	$	11	$	(438	)	$	504,152

For the three months ended March 31, 2019 there were no realized gains. For the fiscal year ended December 31, 2018, there were nominal realized gains. The cost of securities sold is based on the specific identification method.

Unrealized loss positions in the available-for-sale debt securities as of March 31, 2019 and December 31, 2018 reflect temporary impairments that have been in a loss position for less than twelve months and as such are recognized in other comprehensive gain (loss). The fair value of these available-for-sale debt securities in unrealized loss positions was $100.4 million and $403.1 million as of March 31, 2019 and December 31, 2018, respectively.

The following table provides a reconciliation of cash and cash equivalents and restricted cash reported within the consolidated balance sheets that sum to the total of the same such amounts shown in the Company's consolidated statements of cash flows.



(in thousands)	March 31, 2019		December 31, 2018		March 31, 2018		December 31, 2017
Cash and cash equivalents	$	96,349	$	79,749	$	114,322	$	49,060
Restricted cash	2,579		2,626		2,230		2,177
Cash and cash equivalents and restricted cash shown in the statement of cash flows	$	98,928	$	82,375	$	116,552	$	51,237