Guarantor Financial Information (CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS) (Details) - USD ($) $ in Millions	9 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net Cash Provided by Operating Activities	$ 4,405	$ 2,900
Cash flows from investing activities
Capital expenditures - network and other	(2,499)	(1,421)
Capital expenditures - leased devices	(1,787)	(1,530)
Expenditures relating to FCC licenses	(92)	(46)
Proceeds from sales and maturities of short-term investments	7,113	2,649
Payments to Acquire Short-term Investments	(1,842)	(4,998)
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	367	126
Payments for Advance to Affiliate		0
Proceeds from Collection of Advance to Affiliate	0
Proceeds from Related Party Debt		0
Payments for (Proceeds from) Other Investing Activities	16	26
Net Cash Used in Investing Activities	1,276	(5,194)
Cash flows from financing activities
Proceeds from debt and financings	3,073	6,830
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(7,159)	(3,266)
Debt financing costs	19	272
Intercompany dividends paid to parent		0
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Intercompany note advance from parent		0
Repayments of intercompany note advance from parent	0
Other, net	(6)	68
Net cash provided by (used in) financing activities	(4,111)	3,360
Cash and Cash Equivalents, Period Increase (Decrease)	1,570	1,066
Cash and cash equivalents, beginning of period	2,870	2,641
Cash and cash equivalents, end of period	4,440	3,707
Repayments of Related Party Debt		0
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	0	0
Proceeds from sales of assets and FCC Licenses	0	0
Payments for Advance to Affiliate		0
Proceeds from Collection of Advance to Affiliate	0
Proceeds from Related Party Debt		0
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	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
Intercompany dividends paid to parent		0
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Intercompany note advance from parent		0
Repayments of intercompany note advance from parent	0
Other, net	0	0
Net cash provided by (used in) financing activities	0	0
Cash and Cash Equivalents, Period Increase (Decrease)	0	0
Cash and cash equivalents, beginning of period	0	0
Cash and cash equivalents, end of period	0
Repayments of Related Party Debt		0
Guarantor Subsidiaries [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	(1,143)	(1,168)
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	7,113	2,614
Payments to Acquire Short-term Investments	(1,842)	(4,943)
Increase (decrease) due from consolidated affiliate from investing activities	0	6,865
Proceeds from sales of assets and FCC Licenses	0	0
Payments for Advance to Affiliate		(392)
Proceeds from Collection of Advance to Affiliate	575
Proceeds from Related Party Debt		62
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	5,846	4,206
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	(2,530)	(2,000)
Debt financing costs	9	110
Intercompany dividends paid to parent		0
Increase (decrease) due from/to consolidated affiliates from financing activities	(689)	0
Intercompany note advance from parent		0
Repayments of intercompany note advance from parent	0
Other, net	12	35
Net cash provided by (used in) financing activities	(3,216)	(2,075)
Cash and Cash Equivalents, Period Increase (Decrease)	1,487	963
Cash and cash equivalents, beginning of period	2,461	2,154
Cash and cash equivalents, end of period	3,948
Repayments of Related Party Debt		0
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	5,548	4,186
Cash flows from investing activities
Capital expenditures - network and other	(2,499)	(1,421)
Capital expenditures - leased devices	(1,787)	(1,530)
Expenditures relating to FCC licenses	(92)	(46)
Proceeds from sales and maturities of short-term investments	0	35
Payments to Acquire Short-term Investments	0	(55)
Increase (decrease) due from consolidated affiliate from investing activities	689	0
Proceeds from sales of assets and FCC Licenses	367	126
Payments for Advance to Affiliate		0
Proceeds from Collection of Advance to Affiliate	0
Proceeds from Related Party Debt		0
Payments for (Proceeds from) Other Investing Activities	16	26
Net Cash Used in Investing Activities	(3,306)	(2,865)
Cash flows from financing activities
Proceeds from debt and financings	3,073	6,830
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(4,629)	(1,266)
Debt financing costs	10	162
Intercompany dividends paid to parent		(118)
Increase (decrease) due from/to consolidated affiliates from financing activities	0	(6,865)
Intercompany note advance from parent		392
Repayments of intercompany note advance from parent	(575)
Other, net	(18)	33
Net cash provided by (used in) financing activities	(2,159)	(1,218)
Cash and Cash Equivalents, Period Increase (Decrease)	83	103
Cash and cash equivalents, beginning of period	409	487
Cash and cash equivalents, end of period	492
Repayments of Related Party Debt		(62)
Consolidation, Eliminations [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	0	(118)
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	(689)	(6,865)
Proceeds from sales of assets and FCC Licenses	0	0
Payments for Advance to Affiliate		392
Proceeds from Collection of Advance to Affiliate	(575)
Proceeds from Related Party Debt		(62)
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	(1,264)	(6,535)
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
Intercompany dividends paid to parent		118
Increase (decrease) due from/to consolidated affiliates from financing activities	689	6,865
Intercompany note advance from parent		(392)
Repayments of intercompany note advance from parent	575
Other, net	0	0
Net cash provided by (used in) financing activities	1,264	6,653
Cash and Cash Equivalents, Period Increase (Decrease)	0	0
Cash and cash equivalents, beginning of period	0	0
Cash and cash equivalents, end of period	$ 0
Repayments of Related Party Debt		$ 62