Note 3. Cash, Cash Equivalents, Marketable Securities and Restricted Cash

As of March 31, 2018, the Company held $114.3 million in cash and cash equivalents and $490.9 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 income (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. For the first quarter of 2018, only temporary impairment adjustments have been recorded.

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 3 months but less than 1 year are classified as current, while investments that have maturities greater than 1 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, 2018
(in thousands)	Cost		Gross unrealized Gain		Gross unrealized Loss			Fair Value
Cash and cash equivalents	$	114,327	$	—	$	(5	)	$	114,322
Corporate debt securities, current portion	278,281		8		(509		)	277,780
Corporate debt securities, non-current portion	30,400		1		(56		)	30,345
Commercial paper	122,029		—		(169		)	121,860
Asset backed securities	47,924		—		(96		)	47,828
Certificates of deposit	12,759		—		(51		)	12,708
Money market	350		—		—			350

	$	606,070	$	9	$	(886	)	$	605,193

Included in cash and cash equivalents	$	114,327	$	—	$	(5	)	$	114,322
Included in marketable securities, current and non-current	491,743		9		(881		)	490,871

Total cash, cash equivalents and marketable securities	$	606,070	$	9	$	(886	)	$	605,193

	As of December 31, 2017
(in thousands)	Cost		Unrealized Gain		Unrealized Loss			Fair Value
Cash and cash equivalents	$	49,060	$	—	$	—		$	49,060
Corporate debt securities, current portion	199,314		1		(303		)	199,012
Commercial paper	79,878		—		(75		)	79,803
Asset-backed securities	30,346		—		(59		)	30,287
Money market	350		—		—			350
Certificates of deposit	50		—		—			50

	$	358,998	$	1	$	(437	)	$	358,562

Included in cash and cash equivalents	$	49,060	$	—	$	—		$	49,060
Included in marketable securities	309,938		1		(437		)	309,502

Total cash, cash equivalents and marketable securities	$	358,998	$	1	$	(437	)	$	358,562

For the three months ended March 31, 2018 and the fiscal year ended December 31, 2017, there were no realized gains or losses. 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, 2018 and December 31, 2017 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 $478.4 million and $295.1 million as of March 31, 2018 and December 31, 2017, respectively.

The following table provides a reconciliation of cash and cash equivalents, and restricted cash reported within the consolidated balance sheet that sum to the total of the same such amounts shown in the statement of cash flows.

(in thousands)	March 31, 2018		December 31, 2017		March 31, 2017		December 31, 2016

Cash and cash equivalents	$	114,322	$	49,060	$	44,755	$	187,026
Restricted cash	2,230		2,177		388		387

Cash and cash equivalents and restricted cash shown in the statement of cash flows $ 1,791	$	116,552	$	51,237	$	45,143	$	187,413