Investments (Tables)

12 Months Ended

Dec. 31, 2017

Investments [Abstract]

Summary of Investments

The following is a summary of our investments at December 31, 2017 (in thousands):

			Gross Unrealized					Estimated
	Cost		Gains		Losses			Fair Value
Available-for-sale securities (1):
Corporate debt securities	$	132,434	$	—	$	(206	)	$	132,228
Debt securities issued by U.S. government agencies		38,135		—		(59	)		38,076
Total securities with a maturity of one year or less		170,569		—		(265	)		170,304
Corporate debt securities		8,267		—		(35	)		8,232
Debt securities issued by U.S. government agencies		23,264		—		(37	)		23,227
Total securities with a maturity of one to two years		31,531		—		(72	)		31,459
Total available-for-sale securities	$	202,100	$	—	$	(337	)	$	201,763

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(1) Our available-for-sale securities are held at amortized cost.