Guarantor Financial Information (CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS) (Details) - USD ($) $ in Millions	9 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities
Net Cash Provided by Operating Activities	$ 2,603	$ 1,474
Cash flows from investing activities
Capital expenditures - network and other	(3,958)	(3,814)
Capital expenditures - leased devices	(1,724)	(143)
Expenditures relating to FCC licenses	(75)	(121)
Reimbursements relating to FCC licenses	0	95
Proceeds from sales and maturities of short-term investments	377	2,913
Payments to Acquire Short-term Investments	(252)	(1,947)
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	36	114
Sale Leaseback Transaction, Net Proceeds, Investing Activities	1,136	0
Payments for Advance to Affiliate	0
Proceeds from Collection of Advance to Affiliate	0
Payments for (Proceeds from) Other Investing Activities	(25)	(9)
Net Cash Used in Investing Activities	(4,485)	(2,912)
Cash flows from financing activities
Proceeds from debt and financings	755	300
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(727)	(390)
Payments of Debt Issuance Costs	1	37
Proceeds from Issuance of Common Stock	10	50
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
Repayments of intercompany note advance from parent	0
Other, net	10	0
Net cash provided by (used in) financing activities	47	(77)
Cash and Cash Equivalents, Period Increase (Decrease)	(1,835)	(1,515)
Cash and cash equivalents, beginning of period	4,010	4,970
Cash and cash equivalents, end of period	2,175	3,455
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
Reimbursements relating to FCC licenses		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	1	0
Proceeds from sales of assets and FCC Licenses	0	0
Sale Leaseback Transaction, Net Proceeds, Investing Activities	0
Payments for Advance to Affiliate	0
Proceeds from Collection of Advance to Affiliate	0
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	1	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
Payments of Debt Issuance Costs	1	1
Proceeds from Issuance of Common Stock	0	0
Intercompany dividends paid to parent	0	0
Increase (decrease) due from/to consolidated affiliates from financing activities	0	1
Intercompany note advance from parent	0
Repayments of intercompany note advance from parent	0
Other, net	0
Net cash provided by (used in) financing activities	(1)	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	0
Guarantor Subsidiaries [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	(1,029)	(735)
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
Reimbursements relating to FCC licenses		0
Proceeds from sales and maturities of short-term investments	317	2,878
Payments to Acquire Short-term Investments	(197)	(1,857)
Increase (decrease) due from consolidated affiliate from investing activities	(568)	(1,933)
Proceeds from sales of assets and FCC Licenses	0	0
Sale Leaseback Transaction, Net Proceeds, Investing Activities	0
Payments for Advance to Affiliate	(159)
Proceeds from Collection of Advance to Affiliate	54
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	(553)	(912)
Cash flows from financing activities
Proceeds from debt and financings	250	300
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(500)	0
Payments of Debt Issuance Costs	0	5
Proceeds from Issuance of Common Stock	10	50
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
Repayments of intercompany note advance from parent	0
Other, net	0
Net cash provided by (used in) financing activities	(240)	345
Cash and Cash Equivalents, Period Increase (Decrease)	(1,822)	(1,302)
Cash and cash equivalents, beginning of period	3,492	4,125
Cash and cash equivalents, end of period	1,670	2,823
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	3,800	2,409
Cash flows from investing activities
Capital expenditures - network and other	(3,958)	(3,814)
Capital expenditures - leased devices	(1,724)	(143)
Expenditures relating to FCC licenses	(75)	(121)
Reimbursements relating to FCC licenses		95
Proceeds from sales and maturities of short-term investments	60	35
Payments to Acquire Short-term Investments	(55)	(90)
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	36	114
Sale Leaseback Transaction, Net Proceeds, Investing Activities	1,136
Payments for Advance to Affiliate	0
Proceeds from Collection of Advance to Affiliate	0
Payments for (Proceeds from) Other Investing Activities	(25)	(9)
Net Cash Used in Investing Activities	(4,605)	(3,933)
Cash flows from financing activities
Proceeds from debt and financings	505	0
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(227)	(390)
Payments of Debt Issuance Costs	0	31
Proceeds from Issuance of Common Stock	0	0
Intercompany dividends paid to parent	(168)	(200)
Increase (decrease) due from/to consolidated affiliates from financing activities	567	1,932
Intercompany note advance from parent	159
Repayments of intercompany note advance from parent	(54)
Other, net	10
Net cash provided by (used in) financing activities	792	1,311
Cash and Cash Equivalents, Period Increase (Decrease)	(13)	(213)
Cash and cash equivalents, beginning of period	518	845
Cash and cash equivalents, end of period	505	632
Consolidation, Eliminations [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	(168)	(200)
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
Reimbursements relating to FCC licenses		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	567	1,933
Proceeds from sales of assets and FCC Licenses	0	0
Sale Leaseback Transaction, Net Proceeds, Investing Activities	0
Payments for Advance to Affiliate	159
Proceeds from Collection of Advance to Affiliate	(54)
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	672	1,933
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
Payments of Debt Issuance Costs	0	0
Proceeds from Issuance of Common Stock	0	0
Intercompany dividends paid to parent	168	200
Increase (decrease) due from/to consolidated affiliates from financing activities	(567)	(1,933)
Intercompany note advance from parent	(159)
Repayments of intercompany note advance from parent	54
Other, net	0
Net cash provided by (used in) financing activities	(504)	(1,733)
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	$ 0