4.	Financial Instruments

The following tables summarize the estimated fair value and related valuation input hierarchy of our available-for-sale securities as of each period end:

		Total			Total		Total			Total
		Amortized			Unrealized		Unrealized			Estimated
As of December 31, 2016:	Input Level	Cost			Gain		Loss			Fair Value
		(in thousands)
Money market funds	Level 1	$	28,816		$	—	$	—		$	28,816
U.S. Treasury obligations	Level 2		65,403			3		(21	)		65,385
Government agency obligations	Level 2		23,860			5		(5	)		23,860
Corporate debt obligations	Level 2		113,649			8		(172	)		113,485
Commercial paper	Level 2		699			—		—			699
Asset-backed securities	Level 2		13,414			4		(6	)		13,412
Total available-for-sale securities			245,841			20		(204	)		245,657
Less amounts classified as cash equivalents			(37,944	)		—		1			(37,943	)
Amounts classified as short-term securities		$	207,897		$	20	$	(203	)	$	207,714

		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 securities		$	297,254		$	11	$	(529	)	$	296,736

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

	As of December 31, 2016				As of December 31, 2015
	Amortized		Estimated		Amortized		Estimated
	Cost		Fair Value		Cost		Fair Value
	(in thousands)				(in thousands)
Maturing within one year	$	198,022	$	197,956	$	211,311	$	211,059
Maturing in one to five years		47,819		47,701		108,202		107,935
Total available-for-sale securities	$	245,841	$	245,657	$	319,513	$	318,994

As of December 31, 2016, 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, 2016, 2015 and 2014, we did not recognize any other-than-temporary impairment loss.

In addition, restricted cash collateralized by money market funds is a financial asset measured at fair value and is a Level 1 financial instrument under the fair value hierarchy.