Guarantor Financial Information (CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2017	Mar. 31, 2018
Cash flows from operating activities
Net Cash Provided by Operating Activities	$ 2,430	$ 1,924
Cash flows from investing activities
Capital expenditures - network and other	(1,132)	(1,151)
Capital expenditures - leased devices	(1,817)	(1,359)
Expenditures relating to FCC licenses	(59)	(13)
Proceeds from sales and maturities of short-term investments	810	2,594
Payments to Acquire Short-term Investments	(2,464)	(1,499)
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	133	101
Proceeds from Sale and Collection of Receivables	170	375
Proceeds from Collection of Advance to Affiliate	0
Payments for (Proceeds from) Other Investing Activities	(10)	(1)
Net Cash Used in Investing Activities	(4,369)	(953)
Cash flows from financing activities
Proceeds from debt and financings	1,370	902
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(1,415)	(2,121)
Debt financing costs	248	0
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Repayments of intercompany note advance from parent	0
Other, net	(2)	(15)
Net cash provided by (used in) financing activities	(295)	(1,363)
Cash and Cash Equivalents, Period Increase (Decrease)		(392)
Cash, Cash Equivalents, Restricted Cash, Restricted Cash Equivalents, Period Increase (Decrease)	(2,234)	(392)
Cash and cash equivalents, end of period	4,378	2,550	$ 2,942
Call premiums paid on debt redemptions	0	(129)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	4,425	2,550	2,942	$ 6,659
Parent Company [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	0	0
Cash flows from investing activities
Capital expenditures - network and other	0	0
Capital expenditures - leased devices	0	0
Expenditures relating to FCC licenses	0	0
Proceeds from sales and maturities of short-term investments	0	0
Payments to Acquire Short-term Investments	0	0
Increase (decrease) due from consolidated affiliate from investing activities	28	0
Proceeds from sales of assets and FCC Licenses	0	0
Proceeds from Sale and Collection of Receivables	0	0
Proceeds from Collection of Advance to Affiliate	0
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	28	0
Cash flows from financing activities
Proceeds from debt and financings	0	0
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	0	0
Debt financing costs	28
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Repayments of intercompany note advance from parent	0
Other, net	0	0
Net cash provided by (used in) financing activities	(28)	0
Cash and Cash Equivalents, Period Increase (Decrease)		0
Cash, Cash Equivalents, Restricted Cash, Restricted Cash Equivalents, Period Increase (Decrease)	0
Cash and cash equivalents, end of period	0	0	0
Call premiums paid on debt redemptions		0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0			0
Guarantor Subsidiaries [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	(231)	(394)
Cash flows from investing activities
Capital expenditures - network and other	0	0
Capital expenditures - leased devices	0	0
Expenditures relating to FCC licenses	0	0
Proceeds from sales and maturities of short-term investments	810	2,594
Payments to Acquire Short-term Investments	(2,464)	(1,499)
Increase (decrease) due from consolidated affiliate from investing activities	(778)	588
Proceeds from sales of assets and FCC Licenses	0	0
Proceeds from Sale and Collection of Receivables	0	0
Proceeds from Collection of Advance to Affiliate	424
Payments for (Proceeds from) Other Investing Activities	0	2
Net Cash Used in Investing Activities	(2,008)	1,685
Cash flows from financing activities
Proceeds from debt and financings	0	0
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(10)	(1,598)
Debt financing costs	9
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Repayments of intercompany note advance from parent	0
Other, net	(2)	9
Net cash provided by (used in) financing activities	(21)	(1,718)
Cash and Cash Equivalents, Period Increase (Decrease)		(427)
Cash, Cash Equivalents, Restricted Cash, Restricted Cash Equivalents, Period Increase (Decrease)	(2,260)
Cash and cash equivalents, end of period	3,962	2,034	2,461
Call premiums paid on debt redemptions		(129)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	3,962			6,222
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	2,661	2,318
Cash flows from investing activities
Capital expenditures - network and other	(1,132)	(1,151)
Capital expenditures - leased devices	(1,817)	(1,359)
Expenditures relating to FCC licenses	(59)	(13)
Proceeds from sales and maturities of short-term investments	0	0
Payments to Acquire Short-term Investments	0	0
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	133	101
Proceeds from Sale and Collection of Receivables	170	375
Proceeds from Collection of Advance to Affiliate	0
Payments for (Proceeds from) Other Investing Activities	(10)	(3)
Net Cash Used in Investing Activities	(2,715)	(2,050)
Cash flows from financing activities
Proceeds from debt and financings	1,370	902
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(1,405)	(523)
Debt financing costs	211
Increase (decrease) due from/to consolidated affiliates from financing activities	750	(588)
Repayments of intercompany note advance from parent	(424)
Other, net	0	(24)
Net cash provided by (used in) financing activities	80	(233)
Cash and Cash Equivalents, Period Increase (Decrease)		35
Cash, Cash Equivalents, Restricted Cash, Restricted Cash Equivalents, Period Increase (Decrease)	26
Cash and cash equivalents, end of period	416	516	481
Call premiums paid on debt redemptions		0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	463			437
Consolidation, Eliminations [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	0	0
Cash flows from investing activities
Capital expenditures - network and other	0	0
Capital expenditures - leased devices	0	0
Expenditures relating to FCC licenses	0	0
Proceeds from sales and maturities of short-term investments	0	0
Payments to Acquire Short-term Investments	0	0
Increase (decrease) due from consolidated affiliate from investing activities	750	(588)
Proceeds from sales of assets and FCC Licenses	0	0
Proceeds from Sale and Collection of Receivables	0	0
Proceeds from Collection of Advance to Affiliate	(424)
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	326	(588)
Cash flows from financing activities
Proceeds from debt and financings	0	0
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	0	0
Debt financing costs	0
Increase (decrease) due from/to consolidated affiliates from financing activities	(750)	588
Repayments of intercompany note advance from parent	424
Other, net	0	0
Net cash provided by (used in) financing activities	(326)	588
Cash and Cash Equivalents, Period Increase (Decrease)		0
Cash, Cash Equivalents, Restricted Cash, Restricted Cash Equivalents, Period Increase (Decrease)	0
Cash and cash equivalents, end of period	0	0	$ 0
Call premiums paid on debt redemptions		$ 0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 0			$ 0