Revenues from Contracts with Customers - Schedule of Disaggregated Reported Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenue	$ 8,142	$ 8,125
Total service revenue
Disaggregation of Revenue [Line Items]
Total revenue	5,563	5,740
Postpaid
Disaggregation of Revenue [Line Items]
Total revenue	4,199	4,188
Prepaid
Disaggregation of Revenue [Line Items]
Total revenue	843	982
Wholesale, affiliate and other
Disaggregation of Revenue [Line Items]
Total revenue	285	294
Wireline
Disaggregation of Revenue [Line Items]
Total revenue	236	276
Equipment sales
Disaggregation of Revenue [Line Items]
Total revenue	1,220	1,173
Equipment rentals
Disaggregation of Revenue [Line Items]
Total revenue	$ 1,359	$ 1,212