Cash, Cash Equivalents, and Marketable Securities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total cash and cash equivalents	$ 9,558	$ 8,077	$ 8,886	$ 6,179
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	1,028	1,066
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities	2,881	3,280
Investment Type [Member] | Fair Value, Estimate Not Practicable, Carrying (Reported) Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash, time deposits and money market funds	8,530	7,011
US Treasury Securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	0	924
Marketable securities	966	1,634
US Government Agencies Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	300	0
Marketable securities	384	505
Foreign Government Debt Securities [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	703	142
Marketable securities	660	632
Corporate debt [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	25	0
Marketable securities	848	475
Other Debt Obligations [Member] | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Marketable securities	$ 23	$ 34