Guarantor Financial Information (CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Cash flows from operating activities
Net Cash Provided by Operating Activities	$ 128	$ 679
Cash flows from investing activities
Capital expenditures - network and other	(1,802)	(1,246)
Capital expenditures - leased devices	(544)	0
Expenditures relating to FCC licenses	(26)	(41)
Reimbursements relating to FCC licenses	0	95
Proceeds from sales and maturities of short-term investments	138	900
Payments to Acquire Short-term Investments	(175)	(1,002)
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	1	20
Payments for Advance to Affiliate	0
Payments for (Proceeds from) Other Investing Activities	(3)	(3)
Net Cash Used in Investing Activities	(2,411)	(1,277)
Cash flows from financing activities
Proceeds from debt and financings	346	0
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(26)	(210)
Proceeds from Issuance of Common Stock	4	9
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Intercompany note advance from parent	0
Other, net	9	0
Net cash provided by (used in) financing activities	333	(201)
Cash and Cash Equivalents, Period Increase (Decrease)	(1,950)	(799)
Cash and cash equivalents, beginning of period	4,010	4,970
Cash and cash equivalents, end of period	2,060	4,171
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
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
Payments for 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
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	0	0
Proceeds from Issuance of Common Stock	0	0
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Intercompany note advance from parent	0
Other, net	(1)
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	(405)	(429)
Cash flows from investing activities
Capital expenditures - network and other	0	0
Capital expenditures - leased devices	0
Expenditures relating to FCC licenses	0	0
Reimbursements relating to FCC licenses		0
Proceeds from sales and maturities of short-term investments	118	900
Payments to Acquire Short-term Investments	(135)	(1,002)
Increase (decrease) due from consolidated affiliate from investing activities	(1,498)	(58)
Proceeds from sales of assets and FCC Licenses	0	0
Payments for Advance to Affiliate	(55)
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	(1,570)	(160)
Cash flows from financing activities
Proceeds from debt and financings	0
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	0	0
Proceeds from Issuance of Common Stock	4	9
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Intercompany note advance from parent	0
Other, net	0
Net cash provided by (used in) financing activities	4	9
Cash and Cash Equivalents, Period Increase (Decrease)	(1,971)	(580)
Cash and cash equivalents, beginning of period	3,492	4,125
Cash and cash equivalents, end of period	1,521	3,545
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	533	1,108
Cash flows from investing activities
Capital expenditures - network and other	(1,802)	(1,246)
Capital expenditures - leased devices	(544)
Expenditures relating to FCC licenses	(26)	(41)
Reimbursements relating to FCC licenses		95
Proceeds from sales and maturities of short-term investments	20	0
Payments to Acquire Short-term Investments	(40)	0
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	1	20
Payments for Advance to Affiliate	0
Payments for (Proceeds from) Other Investing Activities	(3)	(3)
Net Cash Used in Investing Activities	(2,394)	(1,175)
Cash flows from financing activities
Proceeds from debt and financings	346
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(26)	(210)
Proceeds from Issuance of Common Stock	0	0
Increase (decrease) due from/to consolidated affiliates from financing activities	1,497	58
Intercompany note advance from parent	55
Other, net	10
Net cash provided by (used in) financing activities	1,882	(152)
Cash and Cash Equivalents, Period Increase (Decrease)	21	(219)
Cash and cash equivalents, beginning of period	518	845
Cash and cash equivalents, end of period	539	626
Consolidation, Eliminations [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
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	1,497	58
Proceeds from sales of assets and FCC Licenses	0	0
Payments for Advance to Affiliate	55
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	1,552	58
Cash flows from financing activities
Proceeds from debt and financings	0
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	0	0
Proceeds from Issuance of Common Stock	0	0
Increase (decrease) due from/to consolidated affiliates from financing activities	(1,497)	(58)
Intercompany note advance from parent	(55)
Other, net	0
Net cash provided by (used in) financing activities	(1,552)	(58)
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