Revenues from Contracts with Customers - Schedule of Disaggregated Reported Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenue	$ 7,795	$ 8,433	$ 15,937	$ 16,558
Total service revenue
Disaggregation of Revenue [Line Items]
Total revenue	5,273	5,762	10,836	11,502
Postpaid
Disaggregation of Revenue [Line Items]
Total revenue	4,218	4,255	8,417	8,443
Prepaid
Disaggregation of Revenue [Line Items]
Total revenue	792	954	1,635	1,936
Wholesale, affiliate and other
Disaggregation of Revenue [Line Items]
Total revenue	36	293	321	587
Wireline
Disaggregation of Revenue [Line Items]
Total revenue	227	260	463	536
Equipment sales
Disaggregation of Revenue [Line Items]
Total revenue	1,192	1,418	2,412	2,591
Equipment rentals
Disaggregation of Revenue [Line Items]
Total revenue	$ 1,330	$ 1,253	$ 2,689	$ 2,465