Related Party Transactions (Brightstar) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018
Related Party Transaction [Line Items]
Total revenue	$ 8,433		$ 16,558
Cost of equipment sales	1,517	$ 1,404	2,787	$ 2,949
Brightstar [Member]
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties	209		209		$ 188
Accounts Payable, Related Parties, Current	64		64		$ 88
Total revenue	414	443	829	793
Cost of equipment sales	$ 434	$ 452	$ 866	$ 808