Other Financial Information (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Prepaid Expenses and Other Current Assets [Abstract]
Prepaid expenses	$ 245	$ 206
Materials, supplies and inventory	131	134
Deferred activation and installation charges	109	101
Other	154	106
Total other current assets	639	547
Accounts Payable, Current [Abstract]
Accounts payable	939	1,179
Book overdraft balance	69	56
Capital expenditures incurred but not yet paid	165	196
Other Current Liabilities
Accrued rent	32	31
Legal contingencies	36	30
Other	148	152
Total other current liabilities	$ 216	$ 213