Revenues and Contract Costs - Schedule of Cost Incurred to Obtain or Fulfill Contracts with Customers (Details) $ in Millions	Sep. 30, 2018 USD ($)
Capitalized Contract Cost [Line Items]
Cost incurred to obtain or fulfill contracts with customers	$ 3,510
Corporate
Capitalized Contract Cost [Line Items]
Cost incurred to obtain or fulfill contracts with customers	47
Wireless | Operating Segments
Capitalized Contract Cost [Line Items]
Cost incurred to obtain or fulfill contracts with customers	2,615
Wireline | Operating Segments
Capitalized Contract Cost [Line Items]
Cost incurred to obtain or fulfill contracts with customers	$ 848
Minimum | Corporate
Capitalized Contract Cost [Line Items]
Amortization Period	2 years
Minimum | Wireless | Operating Segments
Capitalized Contract Cost [Line Items]
Amortization Period	2 years
Minimum | Wireline | Operating Segments
Capitalized Contract Cost [Line Items]
Amortization Period	2 years
Maximum | Corporate
Capitalized Contract Cost [Line Items]
Amortization Period	3 years
Maximum | Wireless | Operating Segments
Capitalized Contract Cost [Line Items]
Amortization Period	3 years
Maximum | Wireline | Operating Segments
Capitalized Contract Cost [Line Items]
Amortization Period	5 years
Prepaid expenses and other
Capitalized Contract Cost [Line Items]
Cost incurred to obtain or fulfill contracts with customers	$ 1,839
Other assets
Capitalized Contract Cost [Line Items]
Cost incurred to obtain or fulfill contracts with customers	$ 1,671