Investments (Tables)

6 Months Ended

Jun. 30, 2018

Investments [Abstract]

Fair value of short and long-term investments classified as available for sale

The following tables summarize the unrealized gains and losses and estimated fair value of our investments classified as available-for-sale as of June 30, 2018 and December 31, 2017 (in millions):

	June 30, 2018
	Gross Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Short-term investments:
Restricted cash	$	17	$	—	$	—		$	17
Corporate debt securities	2,376		1		(6		)	2,371
	$	2,393	$	1	$	(6	)	$	2,388
Long-term investments:
Corporate debt securities	4,598		5		(53		)	4,550
	$	4,598	$	5	$	(53	)	$	4,550

	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

Estimated fair values of short and long-term investments classified by date of contractual maturity

The estimated fair values of our short-term and long-term investments classified as available-for-sale by date of contractual maturity as of June 30, 2018 are as follows (in millions):  

	June 30, 2018
One year or less (including restricted cash of $17)	$	2,388
One year through two years	2,127
Two years through three years	1,478
Three years through four years	640
Four years through five years	305
	$	6,938

Schedule of Equity Method Investments

The following table provides a summary of our equity investments as of June 30, 2018 and December 31, 2017 (in millions):

	June 30, 2018		December 31, 2017
Equity investments without readily determinable fair values	$	859	$	872
Equity investments under the equity method of accounting	9		13
Total equity investments	$	868	$	885