Available-for-sale investments

The amortized cost, gross unrealized gains, gross unrealized losses and estimated fair values of available-for-sale investments by type of security were as follows (in millions):



Type of security as of December 31, 2016	Amortized cost		Gross unrealized gains		Gross unrealized losses			Estimated fair value
U.S. Treasury securities	$	6,681	$	1	$	(68	)	$	6,614
Other government-related debt securities:
U.S.	302		—		(3		)	299
Foreign and other	1,784		9		(34		)	1,759
Corporate debt securities:
Financial	8,476		21		(37		)	8,460
Industrial	8,793		59		(63		)	8,789
Other	1,079		5		(7		)	1,077
Residential mortgage-backed securities	1,968		1		(29		)	1,940
Other mortgage- and asset-backed securities	1,731		1		(13		)	1,719
Money market mutual funds	2,782		—		—			2,782
Other short-term interest-bearing securities	4,188		—		—			4,188
Total interest-bearing securities	37,784		97		(254		)	37,627
Equity securities	127		31		(4		)	154
Total available-for-sale investments	$	37,911	$	128	$	(258	)	$	37,781



Type of security as of December 31, 2015	Amortized cost		Gross unrealized gains		Gross unrealized losses			Estimated fair value
U.S. Treasury securities	$	4,298	$	—	$	(24	)	$	4,274
Other government-related debt securities:
U.S.	536		—		(2		)	534
Foreign and other	1,768		7		(36		)	1,739
Corporate debt securities:
Financial	7,904		7		(40		)	7,871
Industrial	7,961		11		(136		)	7,836
Other	905		1		(21		)	885
Residential mortgage-backed securities	1,484		1		(15		)	1,470
Other mortgage- and asset-backed securities	2,524		—		(55		)	2,469
Money market mutual funds	3,370		—		—			3,370
Other short-term interest-bearing securities	528		—		—			528
Total interest-bearing securities	31,278		27		(329		)	30,976
Equity securities	88		48		—			136
Total available-for-sale investments	$	31,366	$	75	$	(329	)	$	31,112

The fair values of available-for-sale investments by classification in the Consolidated Balance Sheets were as follows (in millions):



	December 31,
Classification in the Consolidated Balance Sheets	2016		2015
Cash and cash equivalents	$	2,783	$	3,738
Marketable securities	34,844		27,238
Other assets — noncurrent	154		136
Total available-for-sale investments	$	37,781	$	31,112

Cash and cash equivalents in the table above excludes bank account cash of $458 million and $406 million as of December 31, 2016 and 2015, respectively.

The fair values of available-for-sale interest-bearing security investments by contractual maturity, except for mortgage- and asset-backed securities that do not have a single maturity date, were as follows (in millions):



	December 31,
Contractual maturity	2016		2015
Maturing in one year or less	$	8,393	$	4,578
Maturing after one year through three years	10,404		9,370
Maturing after three years through five years	12,157		9,932
Maturing after five years through ten years	2,974		3,087
Maturing after ten years	40		70
Mortgage- and asset-backed securities	3,659		3,939
Total interest-bearing securities	$	37,627	$	30,976

For the years ended December 31, 2016, 2015 and 2014, realized gains totaled $306 million, $132 million and $149 million, respectively, and realized losses totaled $367 million, $208 million and $150 million, respectively. The cost of securities sold is based on the specific identification method.

The unrealized losses on available-for-sale investments and their related fair values were as follows (in millions):



	Less than 12 months					12 months or greater
Type of security as of December 31, 2016	Fair value		Unrealized losses			Fair value		Unrealized losses
U.S. Treasury securities	$	5,774	$	(68	)	$	—	$	—
Other government-related debt securities:
U.S.	201		(3		)	—		—
Foreign and other	1,192		(34		)	17		—
Corporate debt securities:
Financial	3,975		(37		)	44		—
Industrial	3,913		(61		)	149		(2		)
Other	486		(7		)	7		—
Residential mortgage-backed securities	1,631		(26		)	158		(3		)
Other mortgage- and asset-backed securities	1,087		(10		)	118		(3		)
Equity securities	22		(4		)	—		—
Total	$	18,281	$	(250	)	$	493	$	(8	)



	Less than 12 months					12 months or greater
Type of security as of December 31, 2015	Fair value		Unrealized losses			Fair value		Unrealized losses
U.S. Treasury securities	$	4,196	$	(24	)	$	—	$	—
Other government-related debt securities:
U.S.	494		(2		)	20		—
Foreign and other	1,306		(32		)	56		(4		)
Corporate debt securities:
Financial	5,988		(38		)	228		(2		)
Industrial	5,427		(108		)	679		(28		)
Other	807		(19		)	39		(2		)
Residential mortgage-backed securities	804		(8		)	304		(7		)
Other mortgage- and asset-backed securities	1,834		(19		)	561		(36		)
Total	$	20,856	$	(250	)	$	1,887	$	(79	)

The primary objective of our investment portfolio is to enhance overall returns in an efficient manner while maintaining safety of principal, prudent levels of liquidity and acceptable levels of risk. Our investment policy limits interest-bearing security investments to certain types of debt and money market instruments issued by institutions with primarily investment-grade credit ratings, and it places restrictions on maturities and concentration by asset class and issuer.

We review our available-for-sale investments for other-than-temporary declines in fair value below our cost basis each quarter and whenever events or changes in circumstances indicate that the cost basis of an asset may not be recoverable. The evaluation is based on a number of factors, including the length of time and the extent to which the fair value has been below our cost basis and adverse conditions related specifically to the security, including any changes to the credit rating of the security, and the intent to sell, or whether we will more likely than not be required to sell, the security before recovery of its amortized cost basis. Our assessment of whether a security is other-than-temporarily impaired could change in the future based on new developments or changes in assumptions related to that particular security. As of December 31, 2016 and 2015, we believe the cost bases for our available-for-sale investments were recoverable in all material aspects.