4. CASH, CASH EQUIVALENTS, AND AVAILABLE-FOR-SALE SECURITIES

The fair value and the amortized cost of cash, cash equivalents, and available-for-sale securities by major security type are presented in the tables that follow (in thousands).


	As of June 30, 2018
			Gross		Gross
	Amortized		Unrealized		Unrealized		Estimated
Cash and cash equivalents and available-for-sale securities	Cost		Gains		Losses		Fair Value
Cash and money market funds	$	57,181	$	—	$	—	$	57,181
U.S. Treasury securities		14,513		—		(172)		14,341
Corporate debt securities		52,357		9		(346)		52,020
Total		124,051		9		(518)		123,542
Less amounts classified as cash and cash equivalents		(57,181)		—		—		(57,181)
Total available-for-sale securities	$	66,870	$	9	$	(518)	$	66,361


	As of December 31, 2017
			Gross		Gross
	Amortized		Unrealized		Unrealized		Estimated
Cash and cash equivalents and available-for-sale securities	Cost		Gains		Losses		Fair Value
Cash and money market funds	$	66,392	$	—	$	—	$	66,392
U.S. Treasury securities		21,070		1		(139)		20,932
Corporate debt securities		139,481		16		(486)		139,011
Total		226,943		17		(625)		226,335
Less amounts classified as cash and cash equivalents		(66,392)		—		—		(66,392)
Total available-for-sale securities	$	160,551	$	17	$	(625)	$	159,943

As of June 30, 2018, the Company’s available-for-sale securities had original contractual maturities up to 57 months. However, the Company may sell these securities prior to their stated maturities in response to changes in the availability of and the yield on alternative investments as well as liquidity requirements. As these securities are readily marketable and are viewed by the Company as available to support current operations, securities with maturities beyond 12 months are classified as current assets. Due to their short-term maturities, the Company believes that the fair value of its bank deposits, accounts payable and accrued expenses approximate their carrying value.

Fair Value Measurements

Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. Valuation techniques used to measure fair value must maximize the use of observable inputs and minimize the use of unobservable inputs. Three levels of inputs, of which the first two are considered observable and the last unobservable, may be used to measure fair value. The three levels are:

Level 1 — Quoted prices in active markets for identical assets or liabilities.

Level 2 — Inputs other than Level 1 that are observable, either directly or indirectly, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

The following table represents the fair value hierarchy for our cash equivalents and available-for-sale securities by major security type (in thousands):


	As of June 30, 2018
	Level 1		Level 2		Level 3		Total
Cash and money market funds	$	57,181	$	—	$	—	$	57,181
U.S. Treasury securities		14,341		—		—		14,341
Corporate debt securities		—		52,020		—		52,020
Total	$	71,522	$	52,020	$	—	$	123,542


	As of December 31, 2017
	Level 1		Level 2		Level 3		Total
Cash and money market funds	$	66,392	$	—	$	—	$	66,392
U.S. Treasury securities		20,932		—		—		20,932
Corporate debt securities		—		139,011		—		139,011
Total	$	87,324	$	139,011	$	—	$	226,335