Investments

The following tables summarize the unrealized gains and losses and estimated fair value of our investments classified as available-for-sale as of March 31, 2018 and December 31, 2017 (in millions):



	March 31, 2018
	Gross Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Short-term investments:
Restricted cash	$	19	$	—	$	—		$	19
Corporate debt securities	2,264		1		(5		)	2,260
	$	2,283	$	1	$	(5	)	$	2,279
Long-term investments:
Corporate debt securities	5,064		7		(54		)	5,017
	$	5,064	$	7	$	(54	)	$	5,017



	December 31, 2017
	Gross Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Short-term investments:
Restricted cash	$	20	$	—	$	—		$	20
Corporate debt securities	3,726		1		(4		)	3,723
	$	3,746	$	1	$	(4	)	$	3,743
Long-term investments:
Corporate debt securities	5,458		12		(24		)	5,446
	$	5,458	$	12	$	(24	)	$	5,446

Investment securities in a continuous loss position for greater than 12 months had an estimated fair value of $430 million and an immaterial amount of unrealized losses as of March 31, 2018 and an estimated fair value of $360 million and an immaterial amount of unrealized losses as of December 31, 2017. Refer to “Note 13 - Accumulated Other Comprehensive Income” for amounts reclassified to earnings from unrealized gains and losses.

The estimated fair values of our short-term and long-term investments classified as available-for-sale by date of contractual maturity as of March 31, 2018 are as follows (in millions):



	March 31, 2018
One year or less (including restricted cash of $19)	$	2,279
One year through two years	2,044
Two years through three years	1,723
Three years through four years	735
Four years through five years	515
Five years through six years	—
	$	7,296

Equity investments

Our equity investments are reported in long-term investments on our condensed consolidated balance sheet. The following table provides a summary of our equity investments as of March 31, 2018 and December 31, 2017 (in millions):



	March 31, 2018		December 31, 2017
Equity investments without readily determinable fair values	$	890	$	872
Equity investments under the equity method of accounting	12		13
Total equity investments	$	902	$	885

The changes in equity investments without determinable fair values during the three months ended March 31, 2018 were due to foreign currency translation.