4. Cash, cash equivalents, and available-for-sale marketable securities

Cash equivalents include all highly liquid investments maturing within 90 days from the date of purchase. Investments consist of securities with original maturities greater than 90 days when purchased. The Company classifies these investments as available-for-sale and records them at fair value in the accompanying condensed consolidated balance sheets. Unrealized gains or losses are included in accumulated other comprehensive income (loss). Premiums or discounts from par value are amortized to investment income over the life of the underlying investment.

The Company classifies marketable debt securities with a remaining maturity when purchased of greater than three months as available‑for‑sale. Marketable debt securities with a remaining maturity date greater than one year and marketable equity securities are classified as non‑current where the Company has the intent and ability to hold these securities for at least the next 12 months. Available‑for‑sale debt securities are maintained by an investment manager and consist of U.S. Treasury notes. In 2016, the Company invested in a supplier and received common stock and warrants to purchase common stock in that entity. The common stock is included in non-current marketable securities and the warrants are included in non-current assets.

Realized gains and losses are determined using the specific identification method and are included in other income (expense). If any adjustment to fair value reflects a decline in value of the investment, the Company considers all available evidence to evaluate the extent to which the decline is “other‑than‑temporary” and, if so, recognizes the unrealized loss through a charge to the Company’s statement of operations and comprehensive loss. No other-than-temporary losses have been recognized.

Cash, cash equivalents, and marketable securities included the following at September 30, 2018 and December 31, 2017:


	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
	(in thousands)
As of September 30, 2018
Money market funds included in cash and cash equivalents	$	41,342	$	—	$	—	$	41,342
Marketable securities:
U.S. Treasury notes		137,504		—		45		137,459
Equity securities		1,220		—		420		800
Total marketable securities	$	138,724	$	—	$	465	$	138,259
Total money market funds and marketable securities	$	180,066	$	—	$	465	$	179,601
As of December 31, 2017
Money market funds included in cash and cash equivalents	$	30,469	$	—	$	—	$	30,469
Marketable securities:
U.S. Treasury notes		137,560		—		38		137,522
Equity securities		1,220		—		120		1,100
Total marketable securities	$	138,780	$	—	$	158	$	138,622
Total money market funds and marketable securities	$	169,249	$	—	$	158	$	169,091

All of the Company’s marketable debt securities at September 30, 2018, have a contractual maturity of one year or less.

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


	As of September 30,		As of December 31,
	2018		2017
	(in thousands)
Cash and cash equivalents	$	42,142	$	31,530
Restricted cash included in deposits and other noncurrent assets		735		735
Total cash, cash equivalents, and restricted cash	$	42,877	$	32,265