Guarantor Financial Information (CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2015	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Cash flows from operating activities:
Net cash provided by (used in) operating activities		$ 4,168	$ 3,897	$ 2,450
Cash flows from investing activities:
Capital expenditures - network and other		(1,950)	(4,680)	(5,422)
Capital expenditures - leased devices		(1,925)	(2,292)	(582)
Expenditures relating to FCC licenses		(83)	(98)	(163)
Reimbursements related to FCC licenses		0	0	95
Proceeds from sales and maturities of short-term investments		4,621	418	3,131
Purchases of short-term investments		(10,065)	(252)	(2,077)
Increase (decrease) due from consolidated affiliate from investing activities		0	0	0
Proceeds from sale of assets and FCC licenses		219	62	315
Proceeds from sale-leaseback transaction	$ 1,100	0	1,136	0
Intercompany note advance to consolidated affiliate		0	0	0
Proceeds from intercompany note advance to consolidated affiliate		0	0
Other, net		(42)	(29)	(11)
Net cash used in investing activities		(9,225)	(5,735)	(4,714)
Cash flows from financing activities:
Proceeds from debt and financings		10,966	1,355	1,930
Repayments of debt, financing and capital lease obligations		(5,417)	(899)	(574)
Debt financing costs		(358)	(11)	(87)
Intercompany dividends paid to parent		0	0	0
Increase (decrease) due from/to consolidated affiliates from financing activities		0	0	0
Intercompany note advance from parent		0	0	0
Repayments of intercompany note advance from parent		0	0
Other, net		95	24	35
Net cash provided by (used in) financing activities		5,286	469	1,304
Cash and Cash Equivalents, Period Increase (Decrease)		229	(1,369)	(960)
Cash and cash equivalents, beginning of period		2,641	4,010	4,970
Cash and cash equivalents, end of period		2,870	2,641	4,010
Parent [Member]
Cash flows from operating activities:
Net cash provided by (used in) operating activities		0	0	0
Cash flows from investing activities:
Capital expenditures - network and other		0	0	0
Capital expenditures - leased devices		0	0	0
Expenditures relating to FCC licenses		0	0	0
Reimbursements related to FCC licenses				0
Proceeds from sales and maturities of short-term investments		0	0	0
Purchases of short-term investments		0	0	0
Increase (decrease) due from consolidated affiliate from investing activities		0	1	0
Proceeds from sale of assets and FCC licenses		0	0	0
Proceeds from sale-leaseback transaction			0
Intercompany note advance to consolidated affiliate		0	0	(1,481)
Proceeds from intercompany note advance to consolidated affiliate		0	0
Other, net		0	0	0
Net cash used in investing activities		0	1	(1,481)
Cash flows from financing activities:
Proceeds from debt and financings		0	0	1,500
Repayments of debt, financing and capital lease obligations		0	0	0
Debt financing costs		0	(1)	(21)
Intercompany dividends paid to parent		0	0	0
Increase (decrease) due from/to consolidated affiliates from financing activities		0	0	2
Intercompany note advance from parent		0	0	0
Repayments of intercompany note advance from parent		0	0
Other, net		0	0	0
Net cash provided by (used in) financing activities		0	(1)	1,481
Cash and Cash Equivalents, Period Increase (Decrease)		0	0	0
Cash and cash equivalents, beginning of period		0	0	0
Cash and cash equivalents, end of period		0	0	0
Guarantor Subsidiaries [Member]
Cash flows from operating activities:
Net cash provided by (used in) operating activities		(1,629)	(1,421)	(750)
Cash flows from investing activities:
Capital expenditures - network and other		0	0	0
Capital expenditures - leased devices		0	0	0
Expenditures relating to FCC licenses		0	0	0
Reimbursements related to FCC licenses				0
Proceeds from sales and maturities of short-term investments		4,566	343	3,061
Purchases of short-term investments		(10,010)	(197)	(1,987)
Increase (decrease) due from consolidated affiliate from investing activities		7,097	(36)	(2,425)
Proceeds from sale of assets and FCC licenses		0	0	0
Proceeds from sale-leaseback transaction			0
Intercompany note advance to consolidated affiliate		(414)	(159)	(343)
Proceeds from intercompany note advance to consolidated affiliate		84	372
Other, net		0	0	0
Net cash used in investing activities		1,323	323	(1,694)
Cash flows from financing activities:
Proceeds from debt and financings		4,000	250	300
Repayments of debt, financing and capital lease obligations		(3,250)	(500)	0
Debt financing costs		(187)	0	(5)
Intercompany dividends paid to parent		0	0	0
Increase (decrease) due from/to consolidated affiliates from financing activities		0	0	0
Intercompany note advance from parent		0	0	1,481
Repayments of intercompany note advance from parent		0	0
Other, net		50	10	35
Net cash provided by (used in) financing activities		613	(240)	1,811
Cash and Cash Equivalents, Period Increase (Decrease)		307	(1,338)	(633)
Cash and cash equivalents, beginning of period		2,154	3,492	4,125
Cash and cash equivalents, end of period		2,461	2,154	3,492
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities:
Net cash provided by (used in) operating activities		5,996	5,551	3,700
Cash flows from investing activities:
Capital expenditures - network and other		(1,950)	(4,680)	(5,422)
Capital expenditures - leased devices		(1,925)	(2,292)	(582)
Expenditures relating to FCC licenses		(83)	(98)	(163)
Reimbursements related to FCC licenses				95
Proceeds from sales and maturities of short-term investments		55	75	70
Purchases of short-term investments		(55)	(55)	(90)
Increase (decrease) due from consolidated affiliate from investing activities		0	0	0
Proceeds from sale of assets and FCC licenses		219	62	315
Proceeds from sale-leaseback transaction			1,136
Intercompany note advance to consolidated affiliate		0	0	0
Proceeds from intercompany note advance to consolidated affiliate		0	0
Other, net		(42)	(29)	(11)
Net cash used in investing activities		(3,781)	(5,881)	(5,788)
Cash flows from financing activities:
Proceeds from debt and financings		6,966	1,105	130
Repayments of debt, financing and capital lease obligations		(2,167)	(399)	(574)
Debt financing costs		(171)	(10)	(61)
Intercompany dividends paid to parent		(199)	(233)	(500)
Increase (decrease) due from/to consolidated affiliates from financing activities		(7,097)	35	2,423
Intercompany note advance from parent		414	159	343
Repayments of intercompany note advance from parent		(84)	(372)
Other, net		45	14	0
Net cash provided by (used in) financing activities		(2,293)	299	1,761
Cash and Cash Equivalents, Period Increase (Decrease)		(78)	(31)	(327)
Cash and cash equivalents, beginning of period		487	518	845
Cash and cash equivalents, end of period		409	487	518
Consolidation, Eliminations [Member]
Cash flows from operating activities:
Net cash provided by (used in) operating activities		(199)	(233)	(500)
Cash flows from investing activities:
Capital expenditures - network and other		0	0	0
Capital expenditures - leased devices		0	0	0
Expenditures relating to FCC licenses		0	0	0
Reimbursements related to FCC licenses				0
Proceeds from sales and maturities of short-term investments		0	0	0
Purchases of short-term investments		0	0	0
Increase (decrease) due from consolidated affiliate from investing activities		(7,097)	35	2,425
Proceeds from sale of assets and FCC licenses		0	0	0
Proceeds from sale-leaseback transaction			0
Intercompany note advance to consolidated affiliate		414	159	1,824
Proceeds from intercompany note advance to consolidated affiliate		(84)	(372)
Other, net		0	0	0
Net cash used in investing activities		(6,767)	(178)	4,249
Cash flows from financing activities:
Proceeds from debt and financings		0	0	0
Repayments of debt, financing and capital lease obligations		0	0	0
Debt financing costs		0	0	0
Intercompany dividends paid to parent		199	233	500
Increase (decrease) due from/to consolidated affiliates from financing activities		7,097	(35)	(2,425)
Intercompany note advance from parent		(414)	(159)	(1,824)
Repayments of intercompany note advance from parent		84	372
Other, net		0	0	0
Net cash provided by (used in) financing activities		6,966	411	(3,749)
Cash and Cash Equivalents, Period Increase (Decrease)		0	0	0
Cash and cash equivalents, beginning of period		0	0	0
Cash and cash equivalents, end of period		$ 0	$ 0	$ 0