Financial Instruments (Tables)

12 Months Ended

Dec. 31, 2015

Financial Instruments Disclosure [Abstract]

Fair Value, Assets Measured on Recurring Basis [Table Text Block]

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

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 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