Financial Instruments (Tables)

6 Months Ended

Jun. 30, 2016

Financial Instruments Disclosure [Abstract]

Estimated Fair Value and Related Valuation Input Hierarchy of Financial Assets Comprised Solely of Available-for-Sale Securities Measured on Recurring Basis

Our financial assets are measured at fair value on a recurring basis using the following hierarchy to prioritize valuation inputs, in accordance with applicable GAAP:

Level 1:	Quoted prices in active markets for identical assets or liabilities that we have the ability to access
Level 2:	Observable market based inputs or unobservable inputs that are corroborated by market data such as quoted prices, interest rates and yield curves
Level 3:	Inputs that are unobservable data points that are not corroborated by market data

We review the fair value hierarchy classification on a quarterly basis. Changes in the ability to observe valuation inputs may result in a reclassification of levels of certain securities within the fair value hierarchy. We recognize transfers into and out of levels within the fair value hierarchy in the period in which the actual event or change in circumstances that caused the transfer occurs. There have been no transfers between Level 1, Level 2, and Level 3 in any periods presented.

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 June 30, 2016:	Input Level	Cost			Gain		Loss			Fair Value
		(in thousands)
Money market funds	Level 1	$	21,377		$	—	$	—		$	21,377
U.S. Treasury obligations	Level 2		81,460			114		(3	)		81,571
Government agency obligations	Level 2		29,716			25		(6	)		29,735
Corporate debt obligations	Level 2		154,173			109		(47	)		154,235
Asset-backed securities	Level 2		7,595			4		(2	)		7,597
Total available-for-sale securities			294,321			252		(58	)		294,515
Less amounts classified as cash equivalents			(26,314	)		—		—			(26,314	)
Amounts classified as short-term securities		$	268,007		$	252	$	(58	)	$	268,201
		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

Amortized Cost and Fair Value of Available for Sale Securities by Contractual Maturity

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

	As of June 30, 2016				As of December 31, 2015
	Amortized		Estimated		Amortized		Estimated
	Cost		Fair Value		Cost		Fair Value
	(in thousands)				(in thousands)
Maturing within one year	$	213,377	$	213,431	$	211,311	$	211,059
Maturing in one to five years		80,944		81,084		108,202		107,935
Total available-for-sale securities	$	294,321	$	294,515	$	319,513	$	318,994