Revenue Recognition - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	$ 2,149	$ 2,141		$ 6,380	$ 6,436
Income tax expense	111	168		$ 322	512
Consumer Customers
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
Contract Fulfillment Costs
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Fulfillment costs	58		$ 58	$ 58		$ 61
Voice & Collaboration
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	448	492		1,370	1,504
Transport & Infrastructure
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	739	$ 751		$ 2,216	$ 2,268
Minimum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from contract with customer, contract term				1 year
Customer relationship period				10 years
Maximum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from contract with customer, contract term				7 years
Customer relationship period				20 years
Impact of 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues	(13)			$ (22)
Income tax expense	(4)			(9)
Costs and expenses (less than)	$ 1			$ 1
Impact of 606 | Accounting Standards Update 2014-09 | Consumer
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Operating revenues			10
Impact of 606 | Accounting Standards Update 2014-09 | Contract Fulfillment Costs
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Fulfillment costs			$ 24