Cash, Cash Equivalents, and Marketable Securities (Details) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Total cash and cash equivalents	$ 8,835	$ 8,835	$ 8,077	$ 5,675	$ 8,886
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	669		1,066
Marketable securities	3,101		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	8,166		7,011
U.S. government | Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	112		924
Marketable securities	670		1,634
U.S. government agencies | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	200		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	357		142
Marketable securities	971		632
Corporate debt | Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Cash and cash equivalents	0		0
Marketable securities	1,051		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	$ 25		$ 34