Revenues from Contracts with Customers - Schedule of Disaggregated Reported Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Document Period End Date			Dec. 31, 2019
Total revenue	$ 8,080	$ 8,601	$ 24,017	$ 25,159
Total service revenue
Disaggregation of Revenue [Line Items]
Total revenue	5,416	5,699	16,252	17,201
Postpaid
Disaggregation of Revenue [Line Items]
Total revenue	4,229	4,236	12,646	12,679
Prepaid
Disaggregation of Revenue [Line Items]
Total revenue	740	924	2,375	2,860
Wholesale, affiliate and other
Disaggregation of Revenue [Line Items]
Total revenue	225	294	546	881
Wireline
Disaggregation of Revenue [Line Items]
Total revenue	222	245	685	781
Equipment sales
Disaggregation of Revenue [Line Items]
Total revenue	1,372	1,589	3,784	4,180
Equipment rentals
Disaggregation of Revenue [Line Items]
Total revenue	$ 1,292	$ 1,313	$ 3,981	$ 3,778