Guarantor Financial Information (CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS) (Details) - USD ($) $ in Millions	9 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Condensed Cash Flow Statements, Captions [Line Items]
Acquisition of noncontrolling interest	$ (33)
Cash flows from operating activities
Net Cash Provided by Operating Activities	6,765	$ 7,582
Cash flows from investing activities
Capital expenditures - network and other	(3,360)	(3,814)
Capital expenditures - leased devices	(5,449)	(5,739)
Expenditures relating to FCC licenses	(24)	(145)
Proceeds from sales and maturities of short-term investments	79	6,619
Payments to Acquire Short-term Investments	(74)	(5,152)
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	819	416
Proceeds from intercompany note advance to consolidated affiliate	0
Proceeds from Sale and Collection of Receivables	0	223
Payments for (Proceeds from) Other Investing Activities	(27)	52
Net Cash Used in Investing Activities	(8,031)	(7,430)
Proceeds from Life Insurance Policy	5	110
Cash flows from financing activities
Proceeds from debt and financings	4,731	6,416
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(7,188)	(6,937)
Debt financing costs	(12)	(286)
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Repayments of intercompany note advance from parent	0	0
Other, net	1	0
Net cash provided by (used in) financing activities	(2,530)	(526)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect	(3,796)	(374)
Cash and cash equivalents, end of period	3,179	6,285
Proceeds from Issuance of Common Stock	(29)	281
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	3,267	6,285	$ 7,063	$ 6,659
Proceeds from Related Party Debt		0
Parent Company [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Acquisition of noncontrolling interest	0
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	29	(253)
Proceeds from sales of assets and FCC Licenses	0	0
Proceeds from intercompany note advance to consolidated affiliate	0
Proceeds from Sale and Collection of Receivables		0
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	29	(253)
Proceeds from Life Insurance Policy	0	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	0	(28)
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Repayments of intercompany note advance from parent	0	0
Other, net	0
Net cash provided by (used in) financing activities	(29)	253
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect	0	0
Cash and cash equivalents, end of period	0	0
Proceeds from Issuance of Common Stock	(29)	281
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0		0
Proceeds from Related Party Debt		0
Guarantor Subsidiaries [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Acquisition of noncontrolling interest	(3)
Cash flows from operating activities
Net Cash Provided by Operating Activities	(197)	(408)
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	79	6,619
Payments to Acquire Short-term Investments	74	(5,152)
Increase (decrease) due from consolidated affiliate from investing activities	(3,560)	(1,285)
Proceeds from sales of assets and FCC Licenses	0	0
Proceeds from intercompany note advance to consolidated affiliate	424
Proceeds from Sale and Collection of Receivables		0
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	(3,126)	716
Proceeds from Life Insurance Policy	5	110
Cash flows from financing activities
Proceeds from debt and financings	0	1,100
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(345)	(1,783)
Debt financing costs	(3)	(47)
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Repayments of intercompany note advance from parent	0	0
Other, net	0
Net cash provided by (used in) financing activities	(351)	(730)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect	(3,674)	(422)
Cash and cash equivalents, end of period	2,932	5,800
Proceeds from Issuance of Common Stock	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	2,932		6,606
Proceeds from Related Party Debt		424
Non-Guarantor Subsidiaries [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Acquisition of noncontrolling interest	(30)
Cash flows from operating activities
Net Cash Provided by Operating Activities	6,962	7,990
Cash flows from investing activities
Capital expenditures - network and other	(3,360)	(3,814)
Capital expenditures - leased devices	(5,449)	(5,739)
Expenditures relating to FCC licenses	(24)	(145)
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	819	416
Proceeds from intercompany note advance to consolidated affiliate	0
Proceeds from Sale and Collection of Receivables		223
Payments for (Proceeds from) Other Investing Activities	(27)	52
Net Cash Used in Investing Activities	(8,041)	(9,007)
Proceeds from Life Insurance Policy	0	0
Cash flows from financing activities
Proceeds from debt and financings	4,731	5,316
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(6,843)	(5,154)
Debt financing costs	(9)	(211)
Increase (decrease) due from/to consolidated affiliates from financing activities	3,531	1,538
Repayments of intercompany note advance from parent	(424)	(424)
Other, net	1
Net cash provided by (used in) financing activities	957	1,065
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect	(122)	48
Cash and cash equivalents, end of period	247	485
Proceeds from Issuance of Common Stock	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	335		457
Proceeds from Related Party Debt		0
Consolidation, Eliminations [Member]
Condensed Cash Flow Statements, Captions [Line Items]
Acquisition of noncontrolling interest	0
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	3,531	1,538
Proceeds from sales of assets and FCC Licenses	0	0
Proceeds from intercompany note advance to consolidated affiliate	(424)
Proceeds from Sale and Collection of Receivables		0
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	3,107	1,114
Proceeds from Life Insurance Policy	0	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	0	0
Increase (decrease) due from/to consolidated affiliates from financing activities	(3,531)	(1,538)
Repayments of intercompany note advance from parent	424	424
Other, net	0
Net cash provided by (used in) financing activities	(3,107)	(1,114)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect	0	0
Cash and cash equivalents, end of period	0	0
Proceeds from Issuance of Common Stock	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 0		$ 0
Proceeds from Related Party Debt		$ (424)