Guarantor Financial Information (CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS) (Details) - USD ($) $ in Millions	9 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities
Net Cash Provided by Operating Activities	$ 2,900	$ 2,603
Cash flows from investing activities
Capital expenditures - network and other	(1,421)	(3,958)
Capital expenditures - leased devices	(1,530)	(1,724)
Expenditures relating to FCC licenses	(46)	(75)
Proceeds from sales and maturities of short-term investments	2,649	377
Payments to Acquire Short-term Investments	(4,998)	(252)
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	126	36
Sale Leaseback Transaction, Net Proceeds, Investing Activities	0	1,136
Payments for Advance to Affiliate	0	0
Proceeds from Related Party Debt		0
Proceeds from Collection of Advance to Affiliate	0
Payments for (Proceeds from) Other Investing Activities	26	(25)
Net Cash Used in Investing Activities	(5,194)	(4,485)
Cash flows from financing activities
Proceeds from debt and financings	6,830	755
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(3,266)	(727)
Debt financing costs	272	1
Intercompany dividends paid to parent	0	0
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Intercompany note advance from parent	0	0
Repayments of intercompany note advance from parent	0
Other, net	68	20
Net cash provided by (used in) financing activities	3,360	47
Cash and Cash Equivalents, Period Increase (Decrease)	1,066	(1,835)
Cash and cash equivalents, beginning of period	2,641	4,010
Cash and cash equivalents, end of period	3,707	2,175
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	1
Proceeds from sales of assets and FCC Licenses	0	0
Sale Leaseback Transaction, Net Proceeds, Investing Activities		0
Payments for Advance to Affiliate	0	0
Proceeds from Related Party Debt		0
Proceeds from Collection of Advance to Affiliate	0
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	0	1
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	1
Intercompany dividends paid to parent	0	0
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Intercompany note advance from parent	0	0
Repayments of intercompany note advance from parent	0
Other, net	0	0
Net cash provided by (used in) financing activities	0	(1)
Cash and Cash Equivalents, Period Increase (Decrease)	0	0
Cash and cash equivalents, beginning of period	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	(1,168)	(1,029)
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	2,614	317
Payments to Acquire Short-term Investments	(4,943)	(197)
Increase (decrease) due from consolidated affiliate from investing activities	6,865	(568)
Proceeds from sales of assets and FCC Licenses	0	0
Sale Leaseback Transaction, Net Proceeds, Investing Activities		0
Payments for Advance to Affiliate	(392)	(159)
Proceeds from Related Party Debt		54
Proceeds from Collection of Advance to Affiliate	62
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	4,206	(553)
Cash flows from financing activities
Proceeds from debt and financings	0	250
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(2,000)	(500)
Debt financing costs	110	0
Intercompany dividends paid to parent	0	0
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Intercompany note advance from parent	0	0
Repayments of intercompany note advance from parent	0
Other, net	35	10
Net cash provided by (used in) financing activities	(2,075)	(240)
Cash and Cash Equivalents, Period Increase (Decrease)	963	(1,822)
Cash and cash equivalents, beginning of period	2,154
Cash and cash equivalents, end of period	3,117
Repayments of Related Party Debt		0
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	4,186	3,800
Cash flows from investing activities
Capital expenditures - network and other	(1,421)	(3,958)
Capital expenditures - leased devices	(1,530)	(1,724)
Expenditures relating to FCC licenses	(46)	(75)
Proceeds from sales and maturities of short-term investments	35	60
Payments to Acquire Short-term Investments	(55)	(55)
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	126	36
Sale Leaseback Transaction, Net Proceeds, Investing Activities		1,136
Payments for Advance to Affiliate	0	0
Proceeds from Related Party Debt		0
Proceeds from Collection of Advance to Affiliate	0
Payments for (Proceeds from) Other Investing Activities	26	(25)
Net Cash Used in Investing Activities	(2,865)	(4,605)
Cash flows from financing activities
Proceeds from debt and financings	6,830	505
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(1,266)	(227)
Debt financing costs	162	0
Intercompany dividends paid to parent	(118)	(168)
Increase (decrease) due from/to consolidated affiliates from financing activities	(6,865)	567
Intercompany note advance from parent	392	159
Repayments of intercompany note advance from parent	(62)
Other, net	33	10
Net cash provided by (used in) financing activities	(1,218)	792
Cash and Cash Equivalents, Period Increase (Decrease)	103	(13)
Cash and cash equivalents, beginning of period	487
Cash and cash equivalents, end of period	590
Repayments of Related Party Debt		(54)
Consolidation, Eliminations [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	(118)	(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	0	0
Payments to Acquire Short-term Investments	0	0
Increase (decrease) due from consolidated affiliate from investing activities	(6,865)	567
Proceeds from sales of assets and FCC Licenses	0	0
Sale Leaseback Transaction, Net Proceeds, Investing Activities		0
Payments for Advance to Affiliate	392	159
Proceeds from Related Party Debt		(54)
Proceeds from Collection of Advance to Affiliate	(62)
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	(6,535)	672
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	168
Increase (decrease) due from/to consolidated affiliates from financing activities	6,865	(567)
Intercompany note advance from parent	(392)	(159)
Repayments of intercompany note advance from parent	62
Other, net	0	0
Net cash provided by (used in) financing activities	6,653	(504)
Cash and Cash Equivalents, Period Increase (Decrease)	0	0
Cash and cash equivalents, beginning of period	0
Cash and cash equivalents, end of period	$ 0
Repayments of Related Party Debt		$ 54