AVAILABLE-FOR-SALE DEBT SECURITIES

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



	March 31, 2019									December 31, 2018
	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	$	4,786	$	—	$	(4	)	$	4,782	$	3,978	$	—	$	(9	)	$	3,969
Certificates of deposit	4,211		—		—			4,211		4,361		—		—			4,361
U.S. government agencies securities	1,446		—		(2		)	1,444		943		—		(5		)	938
Municipal debt securities	25		—		—			25		—		—		—			—
Non-U.S. government securities	453		—		(1		)	452		307		—		(2		)	305
Corporate debt securities	12,576		1		(12		)	12,565		13,095		1		(29		)	13,067
Residential mortgage and asset-backed securities	1,040		—		(4		)	1,036		1,532		—		(8		)	1,524
Total	$	24,537	$	1	$	(23	)	$	24,515	$	24,216	$	1	$	(53	)	$	24,164

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



	March 31, 2019		December 31, 2018
Cash and cash equivalents	$	11,317	$	10,592
Short-term marketable securities	10,977		12,149
Long-term marketable securities	2,221		1,423
Total	$	24,515	$	24,164

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



	March 31, 2019
	Amortized Cost		Fair Value
Within one year	$	22,313	$	22,294
After one year through five years	2,175		2,172
After five years through ten years	31		31
After ten years	18		18
Total	$	24,537	$	24,515

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
March 31, 2019
U.S. treasury securities	$	—		$	2,971	$	(4	)	$	917	$	(4	)	$	3,888
U.S. government agencies securities	—			483		(2		)	458		(2		)	941
Non-U.S. government securities	—			100		(1		)	160		(1		)	260
Corporate debt securities	(1		)	1,049		(11		)	2,938		(12		)	3,987
Residential mortgage and asset-backed securities	—			85		(4		)	817		(4		)	902
Total	$	(1	)	$	4,688	$	(22	)	$	5,290	$	(23	)	$	9,978

December 31, 2018
U.S. treasury securities	$	—		$	896	$	(9	)	$	1,383	$	(9	)	$	2,279
U.S. government agencies securities	—			30		(5		)	553		(5		)	583
Non-U.S. government securities	—			86		(2		)	192		(2		)	278
Corporate debt securities	(1		)	1,600		(28		)	4,204		(29		)	5,804
Residential mortgage and asset-backed securities	—			192		(8		)	1,186		(8		)	1,378
Total	$	(1	)	$	2,804	$	(52	)	$	7,518	$	(53	)	$	10,322

We held a total of 915 and 1,348 positions, which were in an unrealized loss position, as of March 31, 2019 and December 31, 2018, respectively.

Based on our review of these securities, we believe we had no other-than-temporary impairments as of March 31, 2019 and December 31, 2018, 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 months ended March 31, 2019 and 2018.