Guarantor Financial Information (CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Cash flows from operating activities
Net Cash Provided by Operating Activities	$ 1,280	$ 542
Cash flows from investing activities
Capital expenditures - network and other	(1,121)	(473)
Capital expenditures - leased devices	(497)	(405)
Expenditures relating to FCC licenses	(13)	(15)
Proceeds from sales and maturities of short-term investments	2,594	0
Payments to Acquire Short-term Investments	(1,499)	(1,304)
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	101	27
Payments for Advance to Affiliate		0
Proceeds from Related Party Debt		0
Payments for (Proceeds from) Other Investing Activities	(3)	(25)
Net Cash Used in Investing Activities	(438)	(2,195)
Cash flows from financing activities
Proceeds from debt and financings	902	3,255
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(2,121)	(294)
Debt financing costs	0	175
Intercompany dividends paid to parent		0
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Intercompany note advance from parent		0
Other, net	(15)	6
Net cash provided by (used in) financing activities	(1,234)	2,792
Cash and Cash Equivalents, Period Increase (Decrease)	(392)	1,139
Cash and cash equivalents, beginning of period	2,870	2,641
Cash and cash equivalents, end of period	2,478	3,780
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
Payments to Acquire Short-term Investments	0	0
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	0	0
Payments for Advance to Affiliate		0
Proceeds from Related Party Debt		0
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	0	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
Debt financing costs		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
Other, net	0	0
Net cash provided by (used in) financing activities	0	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
Repayments of Related Party Debt		0
Guarantor Subsidiaries [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	(521)	(337)
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	2,594
Payments to Acquire Short-term Investments	(1,499)	(1,269)
Increase (decrease) due from consolidated affiliate from investing activities	588	2,924
Proceeds from sales of assets and FCC Licenses	0	0
Payments for Advance to Affiliate		(50)
Proceeds from Related Party Debt		24
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	1,683	1,629
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	(1,598)	0
Debt financing costs		110
Intercompany dividends paid to parent		0
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Intercompany note advance from parent		0
Other, net	9	2
Net cash provided by (used in) financing activities	(1,589)	(108)
Cash and Cash Equivalents, Period Increase (Decrease)	(427)	1,184
Cash and cash equivalents, beginning of period	2,461	2,154
Cash and cash equivalents, end of period	2,034	3,338
Repayments of Related Party Debt		0
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	1,801	943
Cash flows from investing activities
Capital expenditures - network and other	(1,121)	(473)
Capital expenditures - leased devices	(497)	(405)
Expenditures relating to FCC licenses	(13)	(15)
Proceeds from sales and maturities of short-term investments	0
Payments to Acquire Short-term Investments	0	(35)
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	101	27
Payments for Advance to Affiliate		0
Proceeds from Related Party Debt		0
Payments for (Proceeds from) Other Investing Activities	(3)	(25)
Net Cash Used in Investing Activities	(1,533)	(926)
Cash flows from financing activities
Proceeds from debt and financings	902	3,255
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(523)	(294)
Debt financing costs		65
Intercompany dividends paid to parent		(64)
Increase (decrease) due from/to consolidated affiliates from financing activities	(588)	(2,924)
Intercompany note advance from parent		50
Other, net	(24)	4
Net cash provided by (used in) financing activities	(233)	(62)
Cash and Cash Equivalents, Period Increase (Decrease)	35	(45)
Cash and cash equivalents, beginning of period	409	487
Cash and cash equivalents, end of period	444	442
Repayments of Related Party Debt		(24)
Consolidation, Eliminations [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	0	(64)
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	(588)	(2,924)
Proceeds from sales of assets and FCC Licenses	0	0
Payments for Advance to Affiliate		50
Proceeds from Related Party Debt		(24)
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	(588)	(2,898)
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
Intercompany dividends paid to parent		64
Increase (decrease) due from/to consolidated affiliates from financing activities	588	2,924
Intercompany note advance from parent		(50)
Other, net	0	0
Net cash provided by (used in) financing activities	588	2,962
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
Repayments of Related Party Debt		$ 24