Guarantor Financial Information (CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS) (Details) - USD ($) $ in Millions	6 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Cash flows from operating activities
Net Cash Provided by Operating Activities	$ 2,250	$ 1,797
Cash flows from investing activities
Capital expenditures - network and other	(943)	(2,964)
Capital expenditures - leased devices	(763)	(1,117)
Expenditures relating to FCC licenses	(32)	(45)
Proceeds from sales and maturities of short-term investments	1,122	279
Payments to Acquire Short-term Investments	(2,772)	(216)
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	66	4
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	(36)	(21)
Net Cash Used in Investing Activities	(3,358)	(4,080)
Cash flows from financing activities
Proceeds from debt and financings	3,278	434
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(667)	(206)
Debt financing costs	175	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	37	18
Net cash provided by (used in) financing activities	2,473	245
Cash and Cash Equivalents, Period Increase (Decrease)	1,365	(2,038)
Cash and cash equivalents, beginning of period	2,641	4,010
Cash and cash equivalents, end of period	4,006	1,972
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
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	(780)	(759)
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	1,087	219
Payments to Acquire Short-term Investments	(2,737)	(176)
Increase (decrease) due from consolidated affiliate from investing activities	3,883	(1,270)
Proceeds from sales of assets and FCC Licenses	0	0
Payments for Advance to Affiliate	(70)	(55)
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	2,225	(1,228)
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	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	20	8
Net cash provided by (used in) financing activities	(90)	8
Cash and Cash Equivalents, Period Increase (Decrease)	1,355	(1,979)
Cash and cash equivalents, beginning of period	2,154
Cash and cash equivalents, end of period	3,509
Repayments of Related Party Debt		0
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	3,148	2,670
Cash flows from investing activities
Capital expenditures - network and other	(943)	(2,964)
Capital expenditures - leased devices	(763)	(1,117)
Expenditures relating to FCC licenses	(32)	(45)
Proceeds from sales and maturities of short-term investments	35	60
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	66	4
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	(36)	(21)
Net Cash Used in Investing Activities	(1,708)	(4,123)
Cash flows from financing activities
Proceeds from debt and financings	3,278	434
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(667)	(206)
Debt financing costs	65	0
Intercompany dividends paid to parent	(118)	(114)
Increase (decrease) due from/to consolidated affiliates from financing activities	(3,883)	1,269
Intercompany note advance from parent	70	55
Repayments of intercompany note advance from parent	(62)
Other, net	17	10
Net cash provided by (used in) financing activities	(1,430)	1,394
Cash and Cash Equivalents, Period Increase (Decrease)	10	(59)
Cash and cash equivalents, beginning of period	487
Cash and cash equivalents, end of period	497
Repayments of Related Party Debt		(54)
Consolidation, Eliminations [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	(118)	(114)
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	(3,883)	1,269
Proceeds from sales of assets and FCC Licenses	0	0
Payments for Advance to Affiliate	70	55
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	(3,875)	1,270
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	114
Increase (decrease) due from/to consolidated affiliates from financing activities	3,883	(1,269)
Intercompany note advance from parent	(70)	(55)
Repayments of intercompany note advance from parent	62
Other, net	0	0
Net cash provided by (used in) financing activities	3,993	(1,156)
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