AVAILABLE-FOR-SALE DEBT SECURITIES

The following table summarizes our available-for-sale debt securities (in millions):



	September 30, 2018									December 31, 2017
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value		Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
U.S. treasury securities	$	3,090	$	—	$	(15	)	$	3,075	$	4,090	$	—	$	(29	)	$	4,061
Certificates of deposit	4,392		—		—			4,392		5,131		—		—			5,131
U.S. government agencies securities	938		—		(6		)	932		934		—		(8		)	926
Non-U.S. government securities	262		—		(2		)	260		668		—		(4		)	664
Corporate debt securities	12,792		2		(37		)	12,757		14,790		3		(46		)	14,747
Residential mortgage and asset-backed securities	2,049		—		(12		)	2,037		4,072		1		(15		)	4,058
Total	$	23,523	$	2	$	(72	)	$	23,453	$	29,685	$	4	$	(102	)	$	29,587

The following table summarizes the classification of our available-for-sale debt securities in our Condensed Consolidated Balance Sheets (in millions):



	September 30, 2018		December 31, 2017
Cash and cash equivalents	$	7,178	$	481
Short-term marketable securities	13,897		17,922
Long-term marketable securities	2,378		11,184
Total	$	23,453	$	29,587

The following table summarizes our available-for-sale debt securities by contractual maturity (in millions):



	September 30, 2018
	Amortized Cost		Fair Value
Within one year	$	21,130	$	21,075
After one year through five years	2,313		2,299
After five years through ten years	58		57
After ten years	22		22
Total	$	23,523	$	23,453

The following table summarizes our available-for-sale debt securities that were in a continuous unrealized loss position, but were not deemed to be other-than-temporarily impaired (in millions):



	Less Than 12 Months					12 Months or Greater					Total
	Gross Unrealized Losses			Estimated Fair Value		Gross Unrealized Losses			Estimated Fair Value		Gross Unrealized Losses			Estimated Fair Value
September 30, 2018
U.S. treasury securities	$	(1	)	$	1,339	$	(14	)	$	1,613	$	(15	)	$	2,952
U.S. government agencies securities	—			285		(6		)	612		(6		)	897
Non-U.S. government securities	—			—		(2		)	234		(2		)	234
Corporate debt securities	(6		)	2,636		(31		)	3,602		(37		)	6,238
Residential mortgage and asset-backed securities	(1		)	409		(11		)	1,300		(12		)	1,709
Total	$	(8	)	$	4,669	$	(64	)	$	7,361	$	(72	)	$	12,030

December 31, 2017
U.S. treasury securities	$	(2	)	$	821	$	(27	)	$	3,240	$	(29	)	$	4,061
U.S. government agencies securities	(1		)	206		(7		)	700		(8		)	906
Non-U.S. government securities	(1		)	203		(3		)	461		(4		)	664
Corporate debt securities	(14		)	7,674		(32		)	3,561		(46		)	11,235
Residential mortgage and asset-backed securities	(4		)	2,245		(11		)	1,206		(15		)	3,451
Total	$	(22	)	$	11,149	$	(80	)	$	9,168	$	(102	)	$	20,317

We held a total of 1,523 and 2,957 positions, which were in an unrealized loss position, as of September 30, 2018 and December 31, 2017, respectively.

Based on our review of these securities, we believe we had no other-than-temporary impairments as of September 30, 2018 and December 31, 2017, because we do not intend to sell these securities nor do we believe that we will be required to sell these securities before the recovery of their amortized cost basis. Gross realized gains and gross realized losses were not material for the three and nine months ended September 30, 2018 and 2017.