Investments - Available-For-Sale Securities (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
One year or less (including restricted cash of $20)	$ 4,775
One year through two years	1,931
Two years through three years	1,743
Three years through four years	296
Four years through five years	451
Estimated Fair Value	9,196
Restricted cash	19
Short-term Investments
Available-for-sale Securities, Gross Unrealized Gain (Loss) [Abstract]
Gross Amortized Cost	4,754	$ 5,285
Gross Unrealized Gains	23	49
Gross Unrealized Losses	(2)	(1)
Estimated Fair Value	4,775	5,333
Short-term Investments | Restricted cash
Available-for-sale Securities, Gross Unrealized Gain (Loss) [Abstract]
Gross Amortized Cost	20	19
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	20	19
Short-term Investments | Corporate debt securities
Available-for-sale Securities, Gross Unrealized Gain (Loss) [Abstract]
Gross Amortized Cost	4,670	5,203
Gross Unrealized Gains	19	44
Gross Unrealized Losses	(2)	(1)
Estimated Fair Value	4,687	5,246
Short-term Investments | Government and agency securities
Available-for-sale Securities, Gross Unrealized Gain (Loss) [Abstract]
Gross Amortized Cost	64	63
Gross Unrealized Gains	4	5
Gross Unrealized Losses	0	0
Estimated Fair Value	68	68
Long-term Investments
Available-for-sale Securities, Gross Unrealized Gain (Loss) [Abstract]
Gross Amortized Cost	4,411	3,848
Gross Unrealized Gains	18	15
Gross Unrealized Losses	(8)	(12)
Estimated Fair Value	4,421	3,851
Long-term Investments | Corporate debt securities
Available-for-sale Securities, Gross Unrealized Gain (Loss) [Abstract]
Gross Amortized Cost	4,411	3,848
Gross Unrealized Gains	18	15
Gross Unrealized Losses	(8)	(12)
Estimated Fair Value	4,421	$ 3,851
Non-US | Subsidiary | Corporate debt securities
Available-for-sale Securities, Gross Unrealized Gain (Loss) [Abstract]
Gross Unrealized Losses	(20)
Estimated Fair Value	$ 711