We measure certain financial assets and liabilities at fair value. Fair value is determined based upon the exit price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants, as determined by either the principal market or the most advantageous market. Inputs used in the valuation techniques to derive fair values are classified based on a three-level hierarchy, as follows:

Level 1: Observable inputs that reflect quoted prices (unadjusted) for identical assets or liabilities in active markets.

Level 2: Observable inputs other than Level 1 prices such as quoted prices for similar assets or liabilities; quoted prices in markets with insufficient volume or infrequent transactions (less active markets); or model-derived valuations in which all significant inputs are observable or can be derived principally from or 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 asset or liability.

We consider an active market to be one in which transactions for the asset or liability occur with sufficient frequency and volume to provide pricing information on an ongoing basis, and we consider an inactive market to be one in which there are infrequent or few transactions for the asset or liability, the prices are not current, or price quotations vary substantially either over time or among market makers.

The following table presents our financial assets measured and recorded at fair value on a recurring basis using the above input categories:


	As of March 31, 2020
	Level 1		Level 2		Level 3		Total
	(in thousands)
Description:
Assets:
Money market funds	$	185,309	$	—	$	—	$	185,309
Corporate bonds	—		23,579		—		23,579
Commercial paper	—		20,009		—		20,009
Agency bonds	—		3,101		—		3,101
Asset-backed securities	—		2,499		—		2,499
Total assets	$	185,309	$	49,188	$	—	$	234,497


	As of December 31, 2019
	Level 1		Level 2		Level 3		Total
	(in thousands)
Description:
Assets:
Money market funds	$	106,781	$	—	$	—	$	106,781
Corporate bonds	—		60,878		—		60,878
U.S. government agencies	36,979		—		—		36,979
Commercial paper	—		19,966		—		19,966
Agency bonds	—		12,242		—		12,242
Asset-backed securities	—		8,980		—		8,980
Total assets	$	143,760	$	102,066	$	—	$	245,826

As of March 31, 2020, the fair value of our 1.25% convertible senior notes due 2023, as further described in Note 7, Convertible Senior Notes and Capped Calls, was $278.7 million based upon quoted market prices. We consider the fair value of the Notes to be a Level 2 measurement due to limited trading activity of the Notes. We had no other liabilities measured and recorded at fair value on a recurring basis as of March 31, 2020 or December 31, 2019.

Our investments, which are all classified as available-for-sale, consisted of the following:


	As of March 31, 2020
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
	(in thousands)
Description:
Corporate bonds	23,658		22		(101)		23,579
Commercial paper	20,009		—		—		20,009
Agency bonds	3,100		1		—		3,101
Asset-backed securities	2,499		—		—		2,499
Total assets	$	49,266	$	23	$	(101)	$	49,188


	As of December 31, 2019
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
	(in thousands)
Description:
U.S. government agencies	$	36,880	$	99	$	—	$	36,979
Corporate bonds	60,803		77		(2)		60,878
Commercial paper	19,965		1		—		19,966
Agency bonds	12,198		44		—		12,242
Asset-backed securities	8,986		1		(7)		8,980
Total assets	$	138,832	$	222	$	(9)	$	139,045

As of March 31, 2020, our available-for-sale investments had maturities ranging from two to nineteen months. As of December 31, 2019, our available-for-sale investments had maturities ranging from three months to two years.