Guarantor Financial Information (CONDENSED CONSOLIDATING STATEMENT OF CASH FLOWS) (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net Cash Provided by Operating Activities	$ 2,244	$ 2,430
Cash flows from investing activities
Capital expenditures - network and other	(1,189)	(1,132)
Capital expenditures - leased devices	(1,516)	(1,817)
Expenditures relating to FCC licenses	(9)	(59)
Proceeds from sales and maturities of short-term investments	0	810
Payments to Acquire Short-term Investments	67	2,464
Increase (decrease) due from consolidated affiliate from investing activities	0	0
Proceeds from sales of assets and FCC Licenses	182	133
Proceeds from intercompany note advance to consolidated affiliate	0
Proceeds from Sale and Collection of Receivables	0	170
Payments for (Proceeds from) Other Investing Activities	(3)	(10)
Net Cash Used in Investing Activities	(2,468)	(4,369)
Cash flows from financing activities
Proceeds from debt and financings	1,061	1,370
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(2,919)	(1,415)
Debt financing costs	(12)	(248)
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Repayments of intercompany note advance from parent	0	0
Other, net	0	(2)
Net cash provided by (used in) financing activities	(1,887)	(295)
Cash and Cash Equivalents, Period Increase (Decrease)		(2,234)
Cash, Cash Equivalents, Restricted Cash, Restricted Cash Equivalents, Period Increase (Decrease)	(2,111)	(2,234)
Cash and cash equivalents, end of period	4,869
Proceeds from Issuance of Common Stock	(17)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	4,952		$ 7,063	$ 6,659	$ 4,425
Proceeds from 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	17	28
Proceeds from sales of assets and FCC Licenses	0	0
Proceeds from intercompany note advance to consolidated affiliate	0
Proceeds from Sale and Collection of Receivables		0
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	17	28
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	(28)
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Repayments of intercompany note advance from parent	0	0
Other, net		0
Net cash provided by (used in) financing activities	(17)	(28)
Cash and Cash Equivalents, Period Increase (Decrease)		0
Cash, Cash Equivalents, Restricted Cash, Restricted Cash Equivalents, Period Increase (Decrease)	0
Cash and cash equivalents, end of period	0
Proceeds from Issuance of Common Stock	(17)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0		0
Proceeds from Related Party Debt		0
Guarantor Subsidiaries [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	(186)	(231)
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		810
Payments to Acquire Short-term Investments	67	2,464
Increase (decrease) due from consolidated affiliate from investing activities	(2,427)	(778)
Proceeds from sales of assets and FCC Licenses	0	0
Proceeds from intercompany note advance to consolidated affiliate	424
Proceeds from Sale and Collection of Receivables		0
Payments for (Proceeds from) Other Investing Activities	2	0
Net Cash Used in Investing Activities	(1,934)	(2,008)
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	(15)	(10)
Debt financing costs	(2)	(9)
Increase (decrease) due from/to consolidated affiliates from financing activities	0	0
Repayments of intercompany note advance from parent	0	0
Other, net		(2)
Net cash provided by (used in) financing activities	(17)	(21)
Cash and Cash Equivalents, Period Increase (Decrease)		(2,260)
Cash, Cash Equivalents, Restricted Cash, Restricted Cash Equivalents, Period Increase (Decrease)	(2,137)
Cash and cash equivalents, end of period	4,469
Proceeds from Issuance of Common Stock	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	4,469		6,606
Proceeds from Related Party Debt		424
Non-Guarantor Subsidiaries [Member]
Cash flows from operating activities
Net Cash Provided by Operating Activities	2,430	2,661
Cash flows from investing activities
Capital expenditures - network and other	(1,189)	(1,132)
Capital expenditures - leased devices	(1,516)	(1,817)
Expenditures relating to FCC licenses	(9)	(59)
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	182	133
Proceeds from intercompany note advance to consolidated affiliate	0
Proceeds from Sale and Collection of Receivables		170
Payments for (Proceeds from) Other Investing Activities	(5)	(10)
Net Cash Used in Investing Activities	(2,537)	(2,715)
Cash flows from financing activities
Proceeds from debt and financings	1,061	1,370
Repayments of Long-term Debt, Long-term Capital Lease Obligations, and Capital Securities	(2,904)	(1,405)
Debt financing costs	(10)	(211)
Increase (decrease) due from/to consolidated affiliates from financing activities	2,410	750
Repayments of intercompany note advance from parent	(424)	(424)
Other, net		0
Net cash provided by (used in) financing activities	133	80
Cash and Cash Equivalents, Period Increase (Decrease)		26
Cash, Cash Equivalents, Restricted Cash, Restricted Cash Equivalents, Period Increase (Decrease)	26
Cash and cash equivalents, end of period	400
Proceeds from Issuance of Common Stock	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	483		457
Proceeds from Related Party Debt		0
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	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	2,410	750
Proceeds from sales of assets and FCC Licenses	0	0
Proceeds from intercompany note advance to consolidated affiliate	(424)
Proceeds from Sale and Collection of Receivables		0
Payments for (Proceeds from) Other Investing Activities	0	0
Net Cash Used in Investing Activities	1,986	326
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
Increase (decrease) due from/to consolidated affiliates from financing activities	(2,410)	(750)
Repayments of intercompany note advance from parent	424	424
Other, net		0
Net cash provided by (used in) financing activities	(1,986)	(326)
Cash and Cash Equivalents, Period Increase (Decrease)		0
Cash, Cash Equivalents, Restricted Cash, Restricted Cash Equivalents, Period Increase (Decrease)	0
Cash and cash equivalents, end of period	0
Proceeds from Issuance of Common Stock	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 0		$ 0
Proceeds from Related Party Debt		$ (424)