Related Party Transactions (Brightstar) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2018
Related Party Transaction [Line Items]
Total revenue	$ 8,601		$ 25,159
Cost of equipment sales	1,734	$ 1,673	4,521	$ 4,622
Brightstar [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Other Revenues from Transactions with Related Party	52	71	149	100
Accounts Receivable, Related Parties	134		134		$ 188
Accounts Payable, Related Parties, Current	37		37		$ 88
Total revenue	619	639	1,448	1,432
Cost of equipment sales	$ 644	$ 657	$ 1,510	$ 1,465