Short-term Investments (Tables)

12 Months Ended

Dec. 31, 2017

Short-term Investments [Abstract]

Available-For-Sale Securities Reported at Fair Value

The following table summarizes, by major security type, the Company’s assets that were measured at fair value on a recurring basis and were categorized using the fair value hierarchy and where they are classified on the Consolidated Balance Sheets as of December 31, 2016.

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value		Cash and cash equivalents		Short-term investments		Non-current assets (1)
	(in thousands)
Cash	$	1,267,523	$	—	$	—		$	1,267,523	$	1,264,126	$	—	$	3,397
Level 1 securities:
Money market funds	204,967		—		—			204,967		203,450		—		1,517
Level 2 securities:
Corporate debt securities	199,843		110		(731		)	199,222		—		199,222		—
Government securities	35,944		—		(128		)	35,816		—		35,816		—
Certificate of deposit	9,833		—		—			9,833		—		9,833		—
Agency securities	21,563		—		(228		)	21,335		—		21,335		—
Total	$	1,739,673	$	110	$	(1,087	)	$	1,738,696	$	1,467,576	$	266,206	$	4,914

(1) Primarily restricted cash that is related to workers compensation deposits and letter of credit agreements.