3.	Investments and Fair Value Measurements

Investments

The following is a summary of our investments at June 30, 2018 and December 31, 2017 (in thousands):

			Gross Unrealized					Estimated
June 30, 2018	Cost		Gains		Losses			Fair Value
Available-for-sale securities:
Corporate debt securities	$	82,414	$	—	$	(127	)	$	82,287
Debt securities issued by U.S. government agencies		81,893		3		(98	)		81,798
Total securities with a maturity of one year or less		164,307		3		(225	)		164,085

Corporate debt securities		935		—		(9	)		926
Debt securities issued by U.S. government agencies		—		—		—			—
Total securities with a maturity of one to two years		935		—		(9	)		926
Total available-for-sale securities	$	165,242	$	3	$	(234	)	$	165,011

			Gross Unrealized					Estimated
December 31, 2017	Cost		Gains		Losses			Fair Value
Available-for-sale securities:
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

We recorded unrealized losses related to the securities listed above as of June 30, 2018 and December 31, 2017. 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.

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.

Fair Value Measurements

We use a three-tier fair value hierarchy to prioritize the inputs used in our fair value measurements. These tiers include: Level 1, defined as observable inputs such as quoted prices in active markets for identical assets, which includes our money market funds and treasury securities classified as available-for-sale securities; Level 2, defined as inputs other than quoted prices in active markets that are either directly or indirectly observable, which includes our fixed income securities and commercial paper classified as available-for-sale securities; and Level 3, defined as unobservable inputs in which little or no market data exists, therefore requiring us to develop our own assumptions. We have not historically held any Level 3 investments. We recognize transfers between levels of the fair value hierarchy on the date of the event or change in circumstances that caused the transfer.

The following tables present the major security types we held at June 30, 2018 and December 31, 2017 that are regularly measured and carried at fair value. The table segregates each security by the level within the fair value hierarchy of the valuation techniques we utilized to determine the respective securities' fair value (in thousands):

	At June 30, 2018		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)
Money market funds (1)	$	193,752	$	193,752	$	—
Corporate debt securities (2)		89,211		—		89,211
Debt securities issued by U.S. government agencies (3)		81,798		—		81,798
Total	$	364,761	$	193,752	$	171,009

	At December 31, 2017		Quoted Prices in Active Markets (Level 1)		Significant Other Observable Inputs (Level 2)
Money market funds (1)	$	48,430	$	48,430	$	—
Corporate debt securities (3)		140,460		—		140,460
Debt securities issued by U.S. government agencies (3)		61,303		—		61,303
Total	$	250,193	$	48,430	$	201,763

(1)	Included in cash and cash equivalents on our condensed consolidated balance sheets.

(2)	At June 30, 2018, $6.0 million was included in cash and cash equivalents on our condensed consolidated balance sheet, with the difference included in short-term investments on our condensed consolidated balance sheet.

(3)	Included in short-term investments on our condensed consolidated balance sheets.

We did not have any Level 3 investments at June 30, 2018 and December 31, 2017. During the six months ended June 30, 2018 and the year ended December 31, 2017, there were no transfers between Level 1 and Level 2.