Available-for-sale investments

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



	Year ended December 31, 2017
Type of security	Amortized cost		Gross unrealized gains		Gross unrealized losses			Fair value
U.S. Treasury securities	$	8,313	$	1	$	(72	)	$	8,242
Other government-related debt securities:
U.S.	225		—		(2		)	223
Foreign and other	2,415		18		(11		)	2,422
Corporate debt securities:
Financial	10,089		17		(34		)	10,072
Industrial	9,688		34		(52		)	9,670
Other	1,393		3		(6		)	1,390
Residential mortgage-backed securities	2,198		—		(30		)	2,168
Other mortgage- and asset-backed securities	2,312		—		(15		)	2,297
Money market mutual funds	3,245		—		—			3,245
Other short-term interest-bearing securities	1,440		—		—			1,440
Total interest-bearing securities	41,318		73		(222		)	41,169
Equity securities	135		14		—			149
Total available-for-sale investments	$	41,453	$	87	$	(222	)	$	41,318



	Year ended December 31, 2016
Type of security	Amortized cost		Gross unrealized gains		Gross unrealized losses			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

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



	December 31,
Consolidated Balance Sheets location	2017		2016
Cash and cash equivalents	$	3,291	$	2,783
Marketable securities	37,878		34,844
Other assets	149		154
Total available-for-sale investments	$	41,318	$	37,781

Cash and cash equivalents in the above table excludes bank account cash of $509 million and $458 million as of December 31, 2017 and 2016, 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	2017		2016
Maturing in one year or less	$	6,733	$	8,393
Maturing after one year through three years	12,820		10,404
Maturing after three years through five years	13,836		12,157
Maturing after five years through ten years	3,263		2,974
Maturing after ten years	52		40
Mortgage- and asset-backed securities	4,465		3,659
Total interest-bearing securities	$	41,169	$	37,627

For the years ended December 31, 2017, 2016 and 2015, realized gains were $172 million, $306 million and $132 million, respectively, and realized losses were $213 million, $367 million and $208 million, respectively. The cost of securities sold is based on the specific identification method.

Information on the fair values and gross unrealized losses of available-for-sale investments in an unrealized loss position aggregated by type and length of time that the securities have been in a continuous loss position was as follows (in millions):



	Less than 12 months					12 months or greater
Type of security as of December 31, 2017	Fair value		Unrealized losses			Fair value		Unrealized losses
U.S. Treasury securities	$	7,728	$	(70	)	$	195	$	(2	)
Other government-related debt securities:
U.S.	188		(1		)	34		(1		)
Foreign and other	1,163		(9		)	115		(2		)
Corporate debt securities:
Financial	5,928		(28		)	462		(6		)
Industrial	5,760		(43		)	612		(9		)
Other	868		(4		)	117		(2		)
Residential mortgage-backed securities	1,838		(24		)	276		(6		)
Other mortgage- and asset-backed securities	1,777		(12		)	250		(3		)
Total	$	25,250	$	(191	)	$	2,061	$	(31	)



	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	)

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, 2017 and 2016, we believe the cost bases for our available-for-sale investments were recoverable in all material aspects.