Guarantor Financial Information (CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net cash provided by (used in) operating activities	$ 10,062	$ (3,290)	$ (423)
Cash flows from investing activities:
Capital expenditures - network and other	(3,319)	(1,950)	(4,680)
Capital expenditures - leased devices	(7,461)	(4,976)	(5,898)
Expenditures relating to FCC licenses	(115)	(83)	(98)
Proceeds from sales and maturities of short-term investments	7,202	4,621	418
Purchases of short-term investments	(4,112)	(10,065)	(252)
Increase (decrease) due from consolidated affiliate from investing activities	0	0	0
Proceeds from sale of assets and FCC licenses	527	219	62
Proceeds from Sale and Collection of Receivables	1,140	10,498	7,925
Proceeds from sale-leaseback transaction	0	0	1,136
Intercompany note advance to consolidated affiliate	0	0	0
Proceeds from Collection of Advance to Affiliate		0
Proceeds from intercompany note advance to consolidated affiliate			0
Other, net	3	41	(28)
Net cash used in investing activities	(6,135)	(1,695)	(1,415)
Cash flows from financing activities:
Proceeds from debt and financings	8,529	10,966	1,355
Repayments of debt, financing and capital lease obligations	(8,518)	(5,417)	(899)
Debt financing costs	(93)	(358)	(11)
Call premiums paid on debt redemptions	131	0	0
Intercompany dividends paid to parent		0	0
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0	0
Intercompany note advance from parent	0	0	0
Repayments of Related Party Debt			0
Repayments of intercompany note advance from parent		0
Other, net	3	95	24
Net cash provided by (used in) financing activities	(210)	5,286	469
Net increase (decrease) in cash, cash equivalents and restricted cash	3,717	301	(1,369)
Cash, cash equivalents and restricted cash, beginning of period	2,942	2,641	4,010
Cash, cash equivalents and restricted cash, end of period	6,659	2,942	2,641
Parent [Member]
Cash flows from operating activities:
Net cash provided by (used in) operating activities	0	0	0
Cash flows from investing activities:
Capital expenditures - network and other	0	0	0
Capital expenditures - leased devices	0	0	0
Expenditures relating to FCC licenses	0	0	0
Proceeds from sales and maturities of short-term investments	0	0	0
Purchases of short-term investments	0	0	0
Increase (decrease) due from consolidated affiliate from investing activities	0	0	1
Proceeds from sale of assets and FCC licenses	0	0	0
Proceeds from Sale and Collection of Receivables	0	0	0
Proceeds from sale-leaseback transaction			0
Intercompany note advance to consolidated affiliate	(1,476)	0	0
Proceeds from Collection of Advance to Affiliate		0
Proceeds from intercompany note advance to consolidated affiliate			0
Other, net	0	0	0
Net cash used in investing activities	(1,476)	0	1
Cash flows from financing activities:
Proceeds from debt and financings	1,500	0	0
Repayments of debt, financing and capital lease obligations	0	0	0
Debt financing costs	(24)	0	(1)
Call premiums paid on debt redemptions	0
Intercompany dividends paid to parent		0	0
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0	0
Intercompany note advance from parent	0	0	0
Repayments of Related Party Debt			0
Repayments of intercompany note advance from parent		0
Other, net	0	0	0
Net cash provided by (used in) financing activities	1,476	0	(1)
Net increase (decrease) in cash, cash equivalents and restricted cash	0	0	0
Cash, cash equivalents and restricted cash, beginning of period	0	0	0
Cash, cash equivalents and restricted cash, end of period	0	0	0
Guarantor Subsidiaries [Member]
Cash flows from operating activities:
Net cash provided by (used in) operating activities	(828)	(1,640)	(1,422)
Cash flows from investing activities:
Capital expenditures - network and other	0	0	0
Capital expenditures - leased devices	0	0	0
Expenditures relating to FCC licenses	0	0	0
Proceeds from sales and maturities of short-term investments	7,202	4,566	343
Purchases of short-term investments	(4,112)	(10,010)	(197)
Increase (decrease) due from consolidated affiliate from investing activities	0	7,097	(36)
Proceeds from sale of assets and FCC licenses	0	0	0
Proceeds from Sale and Collection of Receivables	0	0	0
Proceeds from sale-leaseback transaction			0
Intercompany note advance to consolidated affiliate	0	(414)	(159)
Proceeds from Collection of Advance to Affiliate		84
Proceeds from intercompany note advance to consolidated affiliate			372
Other, net	2	11	1
Net cash used in investing activities	3,092	1,334	324
Cash flows from financing activities:
Proceeds from debt and financings	0	4,000	250
Repayments of debt, financing and capital lease obligations	(2,587)	(3,250)	(500)
Debt financing costs	(12)	(187)	0
Call premiums paid on debt redemptions	131
Intercompany dividends paid to parent		0	0
Increase (decrease) due from/to consolidated affiliates from financing activities	2,730	0	0
Intercompany note advance from parent	1,476	0	0
Repayments of Related Party Debt			0
Repayments of intercompany note advance from parent		0
Other, net	21	50	10
Net cash provided by (used in) financing activities	1,497	613	(240)
Net increase (decrease) in cash, cash equivalents and restricted cash	3,761	307	(1,338)
Cash, cash equivalents and restricted cash, beginning of period	2,461	2,154	3,492
Cash, cash equivalents and restricted cash, end of period	6,222	2,461	2,154
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities:
Net cash provided by (used in) operating activities	10,890	(1,451)	1,232
Cash flows from investing activities:
Capital expenditures - network and other	(3,319)	(1,950)	(4,680)
Capital expenditures - leased devices	(7,461)	(4,976)	(5,898)
Expenditures relating to FCC licenses	(115)	(83)	(98)
Proceeds from sales and maturities of short-term investments	0	55	75
Purchases of short-term investments	0	(55)	(55)
Increase (decrease) due from consolidated affiliate from investing activities	(2,730)	0	0
Proceeds from sale of assets and FCC licenses	527	219	62
Proceeds from Sale and Collection of Receivables	1,140	10,498	7,925
Proceeds from sale-leaseback transaction			1,136
Intercompany note advance to consolidated affiliate	0	0	0
Proceeds from Collection of Advance to Affiliate		0
Proceeds from intercompany note advance to consolidated affiliate			0
Other, net	1	30	(29)
Net cash used in investing activities	(11,957)	3,738	(1,562)
Cash flows from financing activities:
Proceeds from debt and financings	7,029	6,966	1,105
Repayments of debt, financing and capital lease obligations	(5,931)	(2,167)	(399)
Debt financing costs	(57)	(171)	(10)
Call premiums paid on debt redemptions	0
Intercompany dividends paid to parent		(199)	(233)
Increase (decrease) due from/to consolidated affiliates from financing activities	0	(7,097)	35
Intercompany note advance from parent	0	414	159
Repayments of Related Party Debt			(372)
Repayments of intercompany note advance from parent		(84)
Other, net	(18)	45	14
Net cash provided by (used in) financing activities	1,023	(2,293)	299
Net increase (decrease) in cash, cash equivalents and restricted cash	(44)	(6)	(31)
Cash, cash equivalents and restricted cash, beginning of period	481	487	518
Cash, cash equivalents and restricted cash, end of period	437	481	487
Consolidation, Eliminations [Member]
Cash flows from operating activities:
Net cash provided by (used in) operating activities	0	(199)	(233)
Cash flows from investing activities:
Capital expenditures - network and other	0	0	0
Capital expenditures - leased devices	0	0	0
Expenditures relating to FCC licenses	0	0	0
Proceeds from sales and maturities of short-term investments	0	0	0
Purchases of short-term investments	0	0	0
Increase (decrease) due from consolidated affiliate from investing activities	2,730	(7,097)	35
Proceeds from sale of assets and FCC licenses	0	0	0
Proceeds from Sale and Collection of Receivables	0	0	0
Proceeds from sale-leaseback transaction			0
Intercompany note advance to consolidated affiliate	1,476	414	159
Proceeds from Collection of Advance to Affiliate		(84)
Proceeds from intercompany note advance to consolidated affiliate			(372)
Other, net	0	0	0
Net cash used in investing activities	4,206	(6,767)	(178)
Cash flows from financing activities:
Proceeds from debt and financings	0	0	0
Repayments of debt, financing and capital lease obligations	0	0	0
Debt financing costs	0	0	0
Call premiums paid on debt redemptions	0
Intercompany dividends paid to parent		199	233
Increase (decrease) due from/to consolidated affiliates from financing activities	(2,730)	7,097	(35)
Intercompany note advance from parent	(1,476)	(414)	(159)
Repayments of Related Party Debt			372
Repayments of intercompany note advance from parent		84
Other, net	0	0	0
Net cash provided by (used in) financing activities	(4,206)	6,966	411
Net increase (decrease) in cash, cash equivalents and restricted cash	0	0	0
Cash, cash equivalents and restricted cash, beginning of period	0	0	0
Cash, cash equivalents and restricted cash, end of period	$ 0	$ 0	$ 0