Short-Term and Long-Term Investments - Narrative (Details) - USD ($) $ in Millions	1 Months Ended			12 Months Ended
	Apr. 30, 2017	Mar. 31, 2017	Jul. 31, 2011	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013	Mar. 29, 2017
Gain (Loss) on Investments [Line Items]
Investments, fair value				$ 16	$ 23
Payments to acquire businesses, gross				1
Cash proceeds received				$ 761	$ 0	$ 0
Common stock, shares, issued (in shares)				600,000,000	596,000,000
Market price of common stock				$ 6	$ 6
Cetip
Gain (Loss) on Investments [Line Items]
Investment ownership percentage								12.00%
Consideration transferred, equity			$ 514				$ 514
Impairment loss due to unfavorable foreign exchange rate							190
Investments, fair value				324	432		$ 324	$ 500
Accumulated unrealized gain				176	$ 108
Payments to acquire businesses, gross		$ 319
Cash proceeds received	$ 286			$ 438
Foreign exchange loss	6
Transaction expenses	3
Tax Authority, Brazil | Cetip
Gain (Loss) on Investments [Line Items]
Capital gains tax	$ 26	$ 28
Common Class B3 | Cetip
Gain (Loss) on Investments [Line Items]
Common stock, shares, issued (in shares)	29,623,756
Market price of common stock	$ 181
Proceeds from issuance of common stock	$ 152