Guarantor Financial Information (CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Cash flows from operating activities:
Net cash provided by (used in) operating activities	$ 10,429	$ 10,062	$ (3,290)
Cash flows from investing activities:
Capital expenditures - network and other	(4,963)	(3,319)	(1,950)
Capital expenditures - leased devices	(7,441)	(7,461)	(4,976)
Expenditures relating to FCC licenses	(163)	(115)	(83)
Proceeds from sales and maturities of short-term investments	7,197	7,202	4,621
Purchases of short-term investments	(5,165)	(4,112)	(10,065)
Increase (decrease) due from consolidated affiliate from investing activities	0	0	0
Proceeds from sale of assets and FCC licenses	591	527	219
Proceeds from Sale and Collection of Receivables	223	1,140	10,498
Intercompany note advance to consolidated affiliate		0	0
Proceeds from Life Insurance Policy	110	2	11
Proceeds from Collection of Advance to Affiliate	0
Proceeds from intercompany note advance to consolidated affiliate			0
Other, net	69	1	30
Net cash used in investing activities	(9,542)	(6,135)	(1,695)
Cash flows from financing activities:
Proceeds from debt and financings	9,307	8,529	10,966
Repayments of debt, financing and capital lease obligations	(9,764)	(8,518)	(5,417)
Debt financing costs	(321)	(93)	(358)
Call premiums paid on debt redemptions	0	131	0
Proceeds from Issuance of Common Stock	291	21	50
Intercompany dividends paid to parent			0
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0	0
Intercompany note advance from parent		0	0
Repayments of Related Party Debt			0
Repayments of intercompany note advance from parent	0
Other, net	4	(18)	45
Net cash provided by (used in) financing activities	(483)	(210)	5,286
Net increase (decrease) in cash, cash equivalents and restricted cash	404	3,717	301
Cash, cash equivalents and restricted cash, beginning of period	6,659	2,942	2,641
Cash, cash equivalents and restricted cash, end of period	7,063	6,659	2,942
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	(267)	0	0
Proceeds from sale of assets and FCC licenses	0	0	0
Proceeds from Sale and Collection of Receivables	0	0	0
Intercompany note advance to consolidated affiliate		(1,476)	0
Proceeds from Life Insurance Policy	0	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	(267)	(1,476)	0
Cash flows from financing activities:
Proceeds from debt and financings	0	1,500	0
Repayments of debt, financing and capital lease obligations	0	0	0
Debt financing costs	(28)	(24)	0
Call premiums paid on debt redemptions		0
Proceeds from Issuance of Common Stock	291	0	0
Intercompany dividends paid to parent			0
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0	0
Intercompany note advance from parent		0	0
Repayments of Related Party Debt			0
Repayments of intercompany note advance from parent	0
Other, net	4	0	0
Net cash provided by (used in) financing activities	267	1,476	0
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	(243)	(828)	(1,640)
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	7,197	7,202	4,566
Purchases of short-term investments	(5,165)	(4,112)	(10,010)
Increase (decrease) due from consolidated affiliate from investing activities	(2,060)	0	7,097
Proceeds from sale of assets and FCC licenses	0	0	0
Proceeds from Sale and Collection of Receivables	0	0	0
Intercompany note advance to consolidated affiliate		0	(414)
Proceeds from Life Insurance Policy	110	2	11
Proceeds from Collection of Advance to Affiliate	424
Proceeds from intercompany note advance to consolidated affiliate			84
Other, net	0	0	0
Net cash used in investing activities	506	3,092	1,334
Cash flows from financing activities:
Proceeds from debt and financings	2,000	0	4,000
Repayments of debt, financing and capital lease obligations	(1,798)	(2,587)	(3,250)
Debt financing costs	(81)	(12)	(187)
Call premiums paid on debt redemptions		131
Proceeds from Issuance of Common Stock	0	21	50
Intercompany dividends paid to parent			0
Increase (decrease) due from/to consolidated affiliates from financing activities	0	2,730	0
Intercompany note advance from parent		1,476	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	121	1,497	613
Net increase (decrease) in cash, cash equivalents and restricted cash	384	3,761	307
Cash, cash equivalents and restricted cash, beginning of period	6,222	2,461	2,154
Cash, cash equivalents and restricted cash, end of period	6,606	6,222	2,461
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities:
Net cash provided by (used in) operating activities	10,672	10,890	(1,451)
Cash flows from investing activities:
Capital expenditures - network and other	(4,963)	(3,319)	(1,950)
Capital expenditures - leased devices	(7,441)	(7,461)	(4,976)
Expenditures relating to FCC licenses	(163)	(115)	(83)
Proceeds from sales and maturities of short-term investments	0	0	55
Purchases of short-term investments	0	0	(55)
Increase (decrease) due from consolidated affiliate from investing activities	0	(2,730)	0
Proceeds from sale of assets and FCC licenses	591	527	219
Proceeds from Sale and Collection of Receivables	223	1,140	10,498
Intercompany note advance to consolidated affiliate		0	0
Proceeds from Life Insurance Policy	0	0	0
Proceeds from Collection of Advance to Affiliate	0
Proceeds from intercompany note advance to consolidated affiliate			0
Other, net	69	1	30
Net cash used in investing activities	(11,684)	(11,957)	3,738
Cash flows from financing activities:
Proceeds from debt and financings	7,307	7,029	6,966
Repayments of debt, financing and capital lease obligations	(7,966)	(5,931)	(2,167)
Debt financing costs	(212)	(57)	(171)
Call premiums paid on debt redemptions		0
Proceeds from Issuance of Common Stock	0	0	0
Intercompany dividends paid to parent			(199)
Increase (decrease) due from/to consolidated affiliates from financing activities	2,327	0	(7,097)
Intercompany note advance from parent		0	414
Repayments of Related Party Debt			(84)
Repayments of intercompany note advance from parent	(424)
Other, net	0	(18)	45
Net cash provided by (used in) financing activities	1,032	1,023	(2,293)
Net increase (decrease) in cash, cash equivalents and restricted cash	20	(44)	(6)
Cash, cash equivalents and restricted cash, beginning of period	437	481	487
Cash, cash equivalents and restricted cash, end of period	457	437	481
Consolidation, Eliminations [Member]
Cash flows from operating activities:
Net cash provided by (used in) operating activities	0	0	(199)
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	2,327	2,730	(7,097)
Proceeds from sale of assets and FCC licenses	0	0	0
Proceeds from Sale and Collection of Receivables	0	0	0
Intercompany note advance to consolidated affiliate		1,476	414
Proceeds from Life Insurance Policy	0	0	0
Proceeds from Collection of Advance to Affiliate	(424)
Proceeds from intercompany note advance to consolidated affiliate			(84)
Other, net	0	0	0
Net cash used in investing activities	1,903	4,206	(6,767)
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
Proceeds from Issuance of Common Stock	0	0	0
Intercompany dividends paid to parent			199
Increase (decrease) due from/to consolidated affiliates from financing activities	(2,327)	(2,730)	7,097
Intercompany note advance from parent		(1,476)	(414)
Repayments of Related Party Debt			84
Repayments of intercompany note advance from parent	424
Other, net	0	0	0
Net cash provided by (used in) financing activities	(1,903)	(4,206)	6,966
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