4.	Financial Instruments

The following tables summarize the estimated fair value and related valuation input hierarchy of our financial assets measured on a recurring basis, which were comprised solely of available-for-sale securities as of each period end:

		Total			Total		Total			Total
		Amortized			Unrealized		Unrealized			Estimated
As of December 31, 2015:	Input Level	Cost			Gain		Loss			Fair Value
		(in thousands)
Money market funds	Level 1	$	16,364		$	—	$	—		$	16,364
U.S. Treasury obligations	Level 2		599			—		(1	)		598
Government agency obligations	Level 2		36,480			1		(88	)		36,393
Corporate debt obligations	Level 2		203,767			8		(339	)		203,436
Commercial paper	Level 2		999			—		—			999
Asset-backed securities	Level 2		61,304			2		(102	)		61,204
Total available-for-sale securities			319,513			11		(530	)		318,994
Less amounts classified as cash equivalents			(22,259	)		—		1			(22,258	)
Amounts classified as short-term investments		$	297,254		$	11	$	(529	)	$	296,736

		Total			Total		Total			Total
		Amortized			Unrealized		Unrealized			Estimated
As of December 31, 2014:	Input Level	Cost			Gain		Loss			Fair Value
		(in thousands)
Money market funds	Level 1	$	18,141		$	—	$	—		$	18,141
U.S. Treasury obligations	Level 2		466			—		(1	)		465
Government agency obligations	Level 2		12,519			—		(5	)		12,514
Corporate debt obligations	Level 2		60,052			1		(89	)		59,964
Commercial paper	Level 2		1,200			—		—			1,200
Asset-backed securities	Level 2		11,838			2		(8	)		11,832
Total available-for-sale securities			104,216			3		(103	)		104,116
Less amounts classified as cash equivalents			(21,897	)		—		—			(21,897	)
Amounts classified as short-term investments		$	82,319		$	3	$	(103	)	$	82,219

The amortized cost and fair value of our available-for-sale securities by contractual maturity were as follows:

	As of December 31, 2015				As of December 31, 2014
	Amortized		Estimated		Amortized		Estimated
	Cost		Fair Value		Cost		Fair Value
	(in thousands)				(in thousands)
Maturing within one year	$	211,311	$	211,059	$	78,649	$	78,611
Maturing in one to five years		108,202		107,935		25,567		25,505
Total available-for-sale securities	$	319,513	$	318,994	$	104,216	$	104,116

As of December 31, 2015, certain available-for-sale securities had been in a continuous unrealized loss position, each for less than twelve months. As of this date, no significant facts or circumstances were present to indicate a deterioration in the creditworthiness of the respective issuers, and the Company had no requirement or intention to sell these securities before maturity or recovery of their amortized cost basis. During the years ended December 31, 2015, 2014, and 2013, we did not recognize any other-than-temporary impairment loss.