Cash and Investments

The following table presents cash, cash equivalents, and available-for-sale investments for the periods presented (in thousands):



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Market Value
December 31, 2015:
Cash	$	180,790	$	—	$	—		$	180,790
Cash equivalents:
Money market funds	276,898		—		—			276,898
Commercial paper	12,048		1		—			12,049
US treasury securities	24,999		1		—			25,000
US agency securities	51,500		—		—			51,500
Marketable securities:
Commercial paper	55,350		47		(23		)	55,374
US treasury securities	874,968		20		(959		)	874,029
US agency securities	606,924		17		(739		)	606,202
Corporate debt securities	1,029,463		96		(2,661		)	1,026,898
Municipal securities	10,636		6		—			10,642
Total cash, cash equivalents, and marketable securities	$	3,123,576	$	188	$	(4,382	)	$	3,119,382
December 31, 2014:
Cash	$	213,724	$	—	$	—		$	213,724
Cash equivalents:
Money market funds	95,470		—		—			95,470
Commercial paper	54,340		4		—			54,344
US treasury securities	54,349		—		—			54,349
US agency securities	42,999		1		—			43,000
Marketable securities:
Commercial paper	122,345		33		(7		)	122,371
Certificates of deposit	5,925		2		—			5,927
US treasury securities	1,234,870		64		(366		)	1,234,568
US agency securities	881,843		393		(274		)	881,962
Corporate debt securities	723,412		225		(932		)	722,705
Municipal securities	14,893		4		(8		)	14,889
Total cash, cash equivalents, and marketable securities	$	3,444,170	$	726	$	(1,587	)	$	3,443,309

The following table presents unrealized losses on investments by investment category and length of time the investment has been in a continuous unrealized loss position as of the periods presented (in thousands):



	Less than 12 months					12 Months or Greater				Total
	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
December 31, 2015:
Commercial paper	$	28,482	$	(23	)	$	—	$	—	$	28,482	$	(23	)
US treasury securities	777,237		(959		)	—		—		777,237		(959		)
US agency securities	472,236		(739		)	—		—		472,236		(739		)
Corporate debt securities	943,276		(2,661		)	—		—		943,276		(2,661		)
Total	$	2,221,231	$	(4,382	)	$	—	$	—	$	2,221,231	$	(4,382	)
December 31, 2014:
Commercial paper	$	41,285	$	(7	)	$	—	$	—	$	41,285	$	(7	)
US treasury securities	1,019,771		(366		)	—		—		1,019,771		(366		)
US agency securities	468,106		(274		)	—		—		468,106		(274		)
Corporate debt securities	499,091		(932		)	—		—		499,091		(932		)
Municipal securities	11,042		(8		)	—		—		11,042		(8		)
Total	$	2,039,295	$	(1,587	)	$	—	$	—	$	2,039,295	$	(1,587	)

The following table presents available-for-sale investments by contractual maturity date as of December 31, 2015 (in thousands):



	Amortized Cost		Estimated Fair Market Value
Due in one year or less	$	1,653,937	$	1,652,844
Due after one year through two years	617,493		615,492
Due after two years through three years	305,911		304,809
Total	$	2,577,341	$	2,573,145