Revenue Recognition - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenue	$ 5,778	$ 5,818	$ 5,902	$ 5,945	$ 5,323	$ 4,034	$ 4,090	$ 4,209	$ 23,443	$ 17,656	$ 17,470
Minimum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract term									1 year
Maximum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Contract term									7 years
Consumer Customers | Weighted Average
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Length of customer life									30 months
Business Customer
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Length of customer life									49 months
Business Customer | Minimum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Length of customer life									12 months
Business Customer | Maximum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Length of customer life									60 months