Restructuring Costs - Activity Related to Expenses Incurred and Cash Payments Made (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Beginning Balance	$ 163
Expenses Incurred	34
Cash Payments	(110)
Adjustments	(7)
Restructuring Reserve, Ending Balance	80
Contract termination costs
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Beginning Balance	81
Expenses Incurred	6
Cash Payments	(44)
Adjustments	0
Restructuring Reserve, Ending Balance	43
Severance costs
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Beginning Balance	52
Expenses Incurred	16
Cash Payments	(38)
Adjustments	(5)
Restructuring Reserve, Ending Balance	25
Network decommissioning
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Beginning Balance	30
Expenses Incurred	12
Cash Payments	(28)
Adjustments	(2)
Restructuring Reserve, Ending Balance	$ 12