2.	Investments

As of December 31, 2017, we primarily invested our excess cash in debt instruments of the U.S. Treasury, financial institutions, corporations and U.S. government agencies with strong credit ratings and an investment grade rating at or above A-1, P-1 or F-1 by Moody's, S&P or Fitch, respectively. We have established guidelines relative to diversification and maturities that maintain safety and liquidity. We periodically review and modify these guidelines to maximize trends in yields and interest rates without compromising safety and liquidity.

All of our available-for-sale securities are available to us for use in our current operations. As a result, we categorized all of these securities as current assets even though the stated maturity of some individual securities may be one year or more beyond the balance sheet date.

As of December 31, 2016, we only invested in money market funds which were classified in cash and cash equivalents on our balance sheet.

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.

We believe that the decline in value of these securities is temporary and primarily related to the change in market interest rates since purchase. We believe it is more likely than not that we will be able to hold our debt securities to maturity. Therefore, we anticipate a full recovery of our debt securities' amortized cost basis at maturity.