Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by (used in) operating activities	$ 914	$ 1,252	$ 2,945	$ 2,966
Net cash used in investing activities	(42)	(345)	(810)	(7,012)
Net cash (used in) provided by financing activities	(758)	(349)	(3,025)	(715)
Accounting Standards Update 2014-09 | Contract acquisition costs paid on open contracts
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred contract cost asset					$ 150
Accounting Standards Update 2014-09 | Contracts with promotional bill credits
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred contract cost asset					$ 140
Accounting Standards Update 2016-15 | Adjustments for New Accounting Principle, Early Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by (used in) operating activities	(1,300)	(1,100)	(4,000)	(3,100)
Net cash used in investing activities	1,300	1,100	4,000	3,100
Accounting Standards Update 2016-15 | Adjustment for Debt Prepayments and Debt Extinguishment Costs
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Net cash provided by (used in) operating activities	0	0	(212)	(188)
Net cash (used in) provided by financing activities	$ 0	$ 0	$ (212)	$ (188)