Revenues and Contract Costs - Revenue Performance Obligations (Details) $ in Billions	Jun. 30, 2018 USD ($)
Wireless | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 6.7
Revenue, remaining performance obligation, expected timing of satisfaction, period	6 months
Wireless | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 8.1
Revenue, remaining performance obligation, expected timing of satisfaction, period	1 year
Wireless | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 0.9
Revenue, remaining performance obligation, expected timing of satisfaction, period
Wireline | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-07-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 4.6
Revenue, remaining performance obligation, expected timing of satisfaction, period	6 months
Wireline | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 5.9
Revenue, remaining performance obligation, expected timing of satisfaction, period	1 year
Wireline | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue, remaining performance obligation	$ 2.4
Revenue, remaining performance obligation, expected timing of satisfaction, period