Revenues from Contracts with Customers - Schedule of Disaggregated Reported Revenue (Details) $ in Millions	3 Months Ended
	Jun. 30, 2018 USD ($)
Disaggregation of Revenue [Line Items]
Total revenue	$ 8,125
Total service revenue
Disaggregation of Revenue [Line Items]
Total revenue	5,740
Postpaid
Disaggregation of Revenue [Line Items]
Total revenue	4,188
Prepaid
Disaggregation of Revenue [Line Items]
Total revenue	982
Wireless wholesale and affiliate
Disaggregation of Revenue [Line Items]
Total revenue	294
Wireline
Disaggregation of Revenue [Line Items]
Total revenue	276
Equipment sales
Disaggregation of Revenue [Line Items]
Total revenue	1,173
Equipment rentals
Disaggregation of Revenue [Line Items]
Total revenue	$ 1,212