Related Party Transactions (Brightstar) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended		9 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2019
Related Party Transaction [Line Items]
Document Period End Date							Dec. 31, 2019
Total revenue	$ 8,080		$ 8,601				$ 24,017	$ 25,159
Cost of equipment sales	1,646		1,734				4,346	4,521
Brightstar [Member]
Related Party Transaction [Line Items]
Related Party Transaction, Expenses from Transactions with Related Party	14		18				41	51
Related Party Transaction, Other Revenues from Transactions with Related Party	50		$ 52				134	$ 149
Accounts Receivable, Related Parties	183						183		$ 187
Accounts Payable, Related Parties, Current	$ 74						$ 74		$ 109
Total revenue		$ 394		$ 619	$ 1,090	$ 1,448
Cost of equipment sales		$ 421		$ 644	$ 1,164	$ 1,510