We define fair value as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date.

Our valuation techniques are based on observable and unobservable inputs. Observable inputs reflect readily obtainable data from independent sources, while unobservable inputs reflect our market assumptions. We classify these inputs into the following hierarchy:

The following tables set forth our financial instruments that were measured at fair value on a recurring basis by level within the fair value hierarchy (in thousands):

	June 30, 2019
	Level 1		Level 2		Level 3		Total
Cash and cash equivalents:
Cash and money market funds	$	72,564	$	—	$	—	$	72,564
Total cash and cash equivalents	$	72,564	$	—	$	—	$	72,564
Investments:
Agency securities	$	—	$	21,256	$	—	$	21,256
Corporate securities		—		44,153		—		44,153
Commercial paper		—		11,033		—		11,033
U.S. government securities		—		101,250		—		101,250
Total investments	$	—	$	177,692	$	—	$	177,692

	December 31, 2018
	Level 1		Level 2		Level 3		Total
Cash and cash equivalents:
Cash and money market funds	$	107,511	$	—	$	—	$	107,511
Total cash and cash equivalents	$	107,511	$	—	$	—	$	107,511
Investments:
Agency securities		—		17,352		—		17,352
Corporate securities		—		54,474		—		54,474
Commercial paper		—		5,965		—		5,965
U.S. government securities		—		118,630		—		118,630
Total investments	$	—	$	196,421	$	—	$	196,421

Our valuation techniques used to measure the fair value of money market funds were derived from quoted prices in active markets for identical assets. The valuation techniques used to measure the fair value of investments, all of which have counterparties with high credit ratings, were valued based on quoted market prices or model-driven valuations using significant inputs derived from or corroborated by observable market data. Investments are carried at fair value. During the quarters and six months ended June 30, 2019 and 2018, there were no transfers between Level 1 and Level 2 of the fair value hierarchy.

Available-for-sale investments are carried at fair value and are included in the tables above. The aggregate market value, cost basis, and gross unrealized gains and losses of available-for-sale investments by security type, classified in cash equivalents and investments, as of June 30, 2019 and December 31, 2018, are as follows (in thousands):

	June 30, 2019
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Total Fair Value
Agency securities	$	21,243	$	13	$	—	$	21,256
Corporate securities		44,051		102		—		44,153
Commercial paper		11,033		—		—		11,033
U.S. government securities		101,129		121		—		101,250
Total available-for-sale investments	$	177,456	$	236	$	—	$	177,692

	December 31, 2018
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Total Fair Value
Agency securities	$	17,361	$	—	$	(9	)	$	17,352
Corporate securities		54,536		—		(62	)		54,474
Commercial paper		5,965		—		—			5,965
U.S. government securities		118,667		14		(51	)		118,630
Total available-for-sale investments	$	196,529	$	14	$	(122	)	$	196,421

At June 30, 2019, all of the available-for-sale securities have contractual maturities within eighteen months. We periodically review our available-for-sale investments for other-than-temporary impairment loss. We consider factors such as the duration, severity and the reason for the decline in value, the potential recovery period and our intent to sell. For debt securities, we also consider whether (i) it is more likely than not that we will be required to sell the debt securities before recovery of their amortized cost basis, and (ii) the amortized cost basis cannot be recovered as a result of credit losses. During the quarters and six months ended June 30, 2019 and 2018, we did not recognize any other-than-temporary impairment losses. All marketable securities with unrealized losses have been in a loss position for less than twelve months.

The carrying value of our long-term debt approximates its fair value at each balance sheet date due to its variable interest rate, which approximates a market interest rate.