4. Cash, Cash Equivalents, and Marketable Securities

We consider highly liquid investments with original maturities of three months or less at the time of purchase to be cash equivalents with the exception of time deposits, which are presented as marketable securities. Cash, cash equivalents, and marketable securities consisted of the following at June 30, 2018 and December 31, 2017 (in thousands):



	June 30, 2018		December 31, 2017
Cash and cash equivalents:
Cash	$	1,804,118	$	2,142,949
Money market funds	220,373		125,585
Total cash and cash equivalents	2,024,491		2,268,534
Marketable securities:
Foreign debt	299,961		238,858
Foreign government obligations	117,771		152,850
U.S. debt	38,562		73,671
Time deposits	654,127		255,000
Total marketable securities	1,110,421		720,379
Total cash, cash equivalents, and marketable securities	$	3,134,912	$	2,988,913

The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within our condensed consolidated balance sheets as of June 30, 2018 and December 31, 2017 to the total of such amounts as presented in the condensed consolidated statement of cash flows (in thousands):



	Balance Sheet Line Item	June 30, 2018		December 31, 2017
Cash and cash equivalents	Cash and cash equivalents	$	2,024,491	$	2,268,534
Restricted cash – current (1)	Prepaid expenses and other current assets	6,554		11,120
Restricted cash – noncurrent (1)	Restricted cash and investments	67,380		50,822
Total cash, cash equivalents, and restricted cash		$	2,098,425	$	2,330,476

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(1)	See Note 5. “Restricted Cash and Investments” to our condensed consolidated financial statements for discussion of our “Restricted cash” arrangements.

During the three and six months ended June 30, 2018, we sold marketable securities for proceeds of $10.8 million and realized gains of less than $0.1 million on such sales. During the three and six months ended June 30, 2017, we sold marketable securities for proceeds of $15.1 million and $118.3 million, respectively, and realized gains of less than $0.1 million on such sales. See Note 8. “Fair Value Measurements” to our condensed consolidated financial statements for information about the fair value of our marketable securities.

The following tables summarize the unrealized gains and losses related to our available-for-sale marketable securities, by major security type, as of June 30, 2018 and December 31, 2017 (in thousands):



	As of June 30, 2018
	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value
Foreign debt	$	302,899	$	1	$	2,939	$	299,961
Foreign government obligations	118,992		—		1,221		117,771
U.S. debt	38,607		2		47		38,562
Time deposits	654,127		—		—		654,127
Total	$	1,114,625	$	3	$	4,207	$	1,110,421



	As of December 31, 2017
	Amortized Cost		Unrealized Gains		Unrealized Losses		Fair Value
Foreign debt	$	240,643	$	3	$	1,788	$	238,858
Foreign government obligations	153,999		—		1,149		152,850
U.S. debt	73,746		—		75		73,671
Time deposits	255,000		—		—		255,000
Total	$	723,388	$	3	$	3,012	$	720,379

As of June 30, 2018, we identified nine investments totaling $144.4 million that had been in a loss position for a period of time greater than 12 months with unrealized losses of $1.6 million. As of December 31, 2017, we identified 16 investments totaling $210.3 million that had been in a loss position for a period of time greater than 12 months with unrealized losses of $1.9 million. Such unrealized losses were primarily due to increases in interest rates relative to rates at the time of purchase. Based on the underlying credit quality of the investments, we do not intend to sell these securities prior to the recovery of our cost basis. Therefore, we did not consider these securities to be other-than-temporarily impaired.

The following tables show unrealized losses and fair values for those marketable securities that were in an unrealized loss position as of June 30, 2018 and December 31, 2017, aggregated by major security type and the length of time the marketable securities have been in a continuous loss position (in thousands):



	As of June 30, 2018
	In Loss Position for Less Than 12 Months				In Loss Position for 12 Months or Greater				Total
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
Foreign debt	$	207,034	$	2,040	$	57,927	$	899	$	264,961	$	2,939
Foreign government obligations	31,273		494		86,498		727		117,771		1,221
U.S. debt	23,549		47		—		—		23,549		47
Total	$	261,856	$	2,581	$	144,425	$	1,626	$	406,281	$	4,207



	As of December 31, 2017
	In Loss Position for Less Than 12 Months				In Loss Position for 12 Months or Greater				Total
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
Foreign debt	$	119,869	$	735	$	88,919	$	1,053	$	208,788	$	1,788
Foreign government obligations	31,467		289		121,383		860		152,850		1,149
U.S. debt	73,671		75		—		—		73,671		75
Total	$	225,007	$	1,099	$	210,302	$	1,913	$	435,309	$	3,012

The contractual maturities of our marketable securities as of June 30, 2018 were as follows (in thousands):



	Fair Value
One year or less	$	820,620
One year to two years	128,827
Two years to three years	160,974
Total	$	1,110,421