Fair Value of Financial Instruments (Tables)

3 Months Ended

Mar. 31, 2018

Schedule of Fair Value Measurements of Financial Assets and Liabilities

The following table details the fair value measurements within the fair value hierarchy of the Company’s financial assets and liabilities at March 31, 2018 and December 31, 2017.

	March 31, 2018
	Level 1		Level 2		Level 3		Total
	(in thousands)
Cash equivalents and investments:
Money market funds	$	4,727	$	—	$	—	$	4,727
Commercial paper		—		8,875		—		8,875
Corporate bonds		—		83,159		—		83,159
U.S. government agency obligations		—		2,991		—		2,991
U.S. Treasury securities		—		362,370		—		362,370
Restricted cash:
Certificates of deposit		—		5,968		—		5,968
Total	$	4,727	$	463,363	$	—	$	468,090

	December 31, 2017
	Level 1		Level 2		Level 3		Total
	(in thousands)

Cash equivalents and investments:
Money market funds	$	12,845	$	—	$	—	$	12,845
Commercial paper		—		5,867		—		5,867
Corporate bonds		—		81,668		—		81,668
U.S. government agency obligations		—		3,987		—		3,987
U.S. Treasury securities		—		356,535		—		356,535
Restricted cash:
Certificates of deposit		—		5,105		—		5,105
Total	$	12,845	$	453,162	$	—	$	466,007

Summary of Composition of Short and Long Term Investments

The following tables summarize the composition of our short- and long-term investments at March 31, 2018 and December 31, 2017.

	March 31, 2018
	Amortized Cost		Unrealized Gains		Unrealized Losses			Aggregate Fair Value
	(in thousands)
Commercial paper	$	4,891	$	—	$	(6	)	$	4,885
Corporate bonds		83,648		—		(489	)		83,159
U.S. government agency obligations		3,000		—		(9	)		2,991
U.S. Treasury securities		312,752		—		(308	)		312,444
Total	$	404,291	$	—	$	(812	)	$	403,479

	December 31, 2017
	Amortized Cost		Unrealized Gains		Unrealized Losses			Aggregate Fair Value
	(in thousands)
Commercial paper	$	5,874	$	—	$	(7	)	$	5,867
Corporate bonds		81,947		—		(279	)		81,668
U.S. government agency obligations		4,000		—		(13	)		3,987
U.S. Treasury securities		356,671		8		(144	)		356,535
Total	$	448,492	$	8	$	(443	)	$	448,057

Summary of Contractual Maturities of Short and Long Term Investments

The contractual maturities of short-term and long-term investments held at March 31, 2018 and December 31, 2017 are as follows:

	March 31, 2018				December 31, 2017
	Amortized Cost Basis		Aggregate Fair Value		Amortized Cost Basis		Aggregate Fair Value
	(in thousands)				(in thousands)
Due within one year	$	375,946	$	375,379	$	416,932	$	416,663
Due after 1 year through 2 years		28,345		28,100		31,560		31,394
Total	$	404,291	$	403,479	$	448,492	$	448,057

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

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- Definition

Tabular disclosure of financial instruments measured at fair value, including those classified in shareholders' equity measured on a recurring or nonrecurring basis. Disclosures include, but are not limited to, fair value measurements recorded and the reasons for the measurements, level within the fair value hierarchy in which the fair value measurements are categorized and transfers between levels 1 and 2. Nonrecurring fair value measurements are those that are required or permitted in the statement of financial position in particular circumstances.

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