Guarantor Financial Information (CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Cash flows from operating activities
Net Cash Provided by Operating Activities	$ 542	$ 128
Cash flows from investing activities
Capital expenditures - network and other	(473)	(1,802)
Capital expenditures - leased devices	(405)	(544)
Expenditures relating to FCC licenses	(15)	(26)
Proceeds from sales and maturities of short-term investments	0	138
Payments to Acquire Short-term Investments	(1,304)	(175)
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	27	1
Payments for Advance to Affiliate	0	0
Proceeds from Collection of Advance to Affiliate	0
Payments for (Proceeds from) Other Investing Activities	(25)	(3)
Net Cash Used in Investing Activities	(2,195)	(2,411)
Cash flows from financing activities
Proceeds from debt and financings	3,255	346
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(294)	(26)
Payments of Debt Issuance Costs	175	1
Intercompany dividends paid to parent	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	6	14
Net cash provided by (used in) financing activities	2,792	333
Cash and Cash Equivalents, Period Increase (Decrease)	1,139	(1,950)
Cash and cash equivalents, beginning of period	2,641	4,010
Cash and cash equivalents, end of period	3,780	2,060
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
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
Payments for Advance to Affiliate	0	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
Payments of Debt Issuance Costs	0	1
Intercompany dividends paid to parent	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
Guarantor Subsidiaries [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	(337)	(405)
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		118
Payments to Acquire Short-term Investments	(1,269)	(135)
Increase (decrease) due from consolidated affiliate from investing activities	2,924	(1,498)
Proceeds from sales of assets and FCC Licenses	0	0
Payments for Advance to Affiliate	(50)	(55)
Proceeds from Collection of Advance to Affiliate	24
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	1,629	(1,570)
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	110	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	0
Repayments of intercompany note advance from parent	0
Other, net	2	4
Net cash provided by (used in) financing activities	(108)	4
Cash and Cash Equivalents, Period Increase (Decrease)	1,184	(1,971)
Cash and cash equivalents, beginning of period	2,154
Cash and cash equivalents, end of period	3,338
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	943	533
Cash flows from investing activities
Capital expenditures - network and other	(473)	(1,802)
Capital expenditures - leased devices	(405)	(544)
Expenditures relating to FCC licenses	(15)	(26)
Proceeds from sales and maturities of short-term investments		20
Payments to Acquire Short-term Investments	(35)	(40)
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	27	1
Payments for Advance to Affiliate	0	0
Proceeds from Collection of Advance to Affiliate	0
Payments for (Proceeds from) Other Investing Activities	(25)	(3)
Net Cash Used in Investing Activities	(926)	(2,394)
Cash flows from financing activities
Proceeds from debt and financings	3,255	346
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(294)	(26)
Payments of Debt Issuance Costs	65	0
Intercompany dividends paid to parent	(64)
Increase (decrease) due from/to consolidated affiliates from financing activities	(2,924)	1,497
Intercompany note advance from parent	50	55
Repayments of intercompany note advance from parent	(24)
Other, net	4	10
Net cash provided by (used in) financing activities	(62)	1,882
Cash and Cash Equivalents, Period Increase (Decrease)	(45)	21
Cash and cash equivalents, beginning of period	487
Cash and cash equivalents, end of period	442
Consolidation, Eliminations [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	(64)	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
Payments to Acquire Short-term Investments	0	0
Increase (decrease) due from consolidated affiliate from investing activities	(2,924)	1,497
Proceeds from sales of assets and FCC Licenses	0	0
Payments for Advance to Affiliate	50	55
Proceeds from Collection of Advance to Affiliate	(24)
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	(2,898)	1,552
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
Intercompany dividends paid to parent	64
Increase (decrease) due from/to consolidated affiliates from financing activities	2,924	(1,497)
Intercompany note advance from parent	(50)	(55)
Repayments of intercompany note advance from parent	24
Other, net	0	0
Net cash provided by (used in) financing activities	2,962	(1,552)
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