AVAILABLE-FOR-SALE DEBT SECURITIES

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



	June 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
Corporate debt securities	$	12,144	$	3	$	(55	)	$	12,092	$	14,790	$	3	$	(46	)	$	14,747
U.S. treasury securities	3,412		—		(21		)	3,391		4,090		—		(29		)	4,061
Residential mortgage and asset-backed securities	2,674		—		(17		)	2,657		4,072		1		(15		)	4,058
U.S. government agencies securities	880		—		(8		)	872		934		—		(8		)	926
Certificates of deposit	3,145		—		—			3,145		5,131		—		—			5,131
Non-U.S. government securities	407		—		(3		)	404		668		—		(4		)	664
Total	$	22,662	$	3	$	(104	)	$	22,561	$	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):



	June 30, 2018		December 31, 2017
Cash and cash equivalents	$	4,139	$	481
Short-term marketable securities	12,683		17,922
Long-term marketable securities	5,739		11,184
Total	$	22,561	$	29,587

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



	June 30, 2018
	Amortized Cost		Fair Value
Within one year	$	16,861	$	16,822
After one year through five years	5,710		5,649
After five years through ten years	67		66
After ten years	24		24
Total	$	22,662	$	22,561

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
June 30, 2018
Corporate debt securities	$	(15	)	$	3,774	$	(40	)	$	3,449	$	(55	)	$	7,223
U.S. treasury securities	(1		)	239		(20		)	2,482		(21		)	2,721
Residential mortgage and asset-backed securities	(5		)	1,176		(12		)	1,167		(17		)	2,343
U.S. government agencies securities	—			46		(8		)	687		(8		)	733
Certificates of deposit	—			71		—			—		—			71
Non-U.S. government securities	—			64		(3		)	323		(3		)	387
Total	$	(21	)	$	5,370	$	(83	)	$	8,108	$	(104	)	$	13,478

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

We held a total of 1,866 and 2,957 positions as of June 30, 2018 and December 31, 2017, respectively, which were in an unrealized loss position.

Based on our review of these securities, we believe we had no other-than-temporary impairments as of June 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 six months ended June 30, 2018 and 2017.