Other Financial Information (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Prepaid Expenses and Other Current Assets [Abstract]
Prepaid expenses	$ 329	$ 294
Income tax receivable	427	258
Materials, supplies and inventory	107	128
Deferred activation and installation charges	95	128
Deferred commissions	148	0
Other	154	133
Total other current assets	1,260	941
Other Current Liabilities
Accrued rent	30	34
Litigation contingencies	30	45
Other	298	265
Total other current liabilities	358	344
Accounts Payable, Current [Abstract]
Book overdraft balance	$ 48	$ 36