Other Financial Information (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Prepaid Expenses and Other Current Assets [Abstract]
Prepaid expenses	$ 353		$ 294
Income tax receivable	256		258
Materials, supplies and inventory	111		128
Deferred activation and installation charges	157		128
Deferred commissions	134		0
Other	94		133
Total other current assets	1,105		941
Other Current Liabilities
Accrued rent	28		34
Legal contingencies	46		45
Other	354		265
Total other current liabilities	428		344
Accounts Payable, Current [Abstract]
Book overdraft balance	24		$ 36
Capital expenditures incurred but not yet paid	$ 186	$ 225