Guarantor Financial Information (CONDENSED CONSOLIDATING BALANCE SHEET) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016
Current Assets
Cash and Cash Equivalents	$ 4,440	$ 2,870	$ 3,707	$ 3,707	$ 2,641
Short-term Investments	173	5,444
Accounts and notes receivable, net	3,917	4,138
Due from Affiliate, Current	0
Device and accessory inventory	1,009	1,064
Prepaid expenses and other current assets	626	601
Total current assets	10,165	14,117
Investments in Subsidiaries	0	0
Property, plant and equipment, net	19,712	19,209
Due from Consolidated Affiliate	0	0
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	6,586	6,579
FCC licenses and other	41,222	40,585
Definite-lived intangible assets, net	2,667	3,320
Other assets	1,067	1,313
Total assets	81,419	85,123
Current Liabilities
Accounts Payable, Current	3,176	3,281
Accrued expenses and other current liabilities	3,859	4,141
Current portion of long-term debt, financing and capital lease obligations	4,036	5,036
Due to Affiliate, Current	0
Total current liabilities	11,071	12,458
Long-term debt, financing and capital lease obligations	32,825	35,878
Deferred tax liabilities	7,709	14,416
Note due to consolidated affiliate	0	0
Other liabilities	3,509	3,563
Due to affiliate	0	0
Total liabilities	55,114	66,315
Commitments and contingencies
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	26,305	18,808
Stockholders' Equity Attributable to Noncontrolling Interest	70	0
Total stockholders' equity	26,235	18,808
Liabilities and Equity	81,419	85,123
Due from Affiliate, Noncurrent	0	0
Consolidation, Eliminations [Member]
Current Assets
Cash and Cash Equivalents	0	0		0	0
Short-term Investments	0	0
Accounts and notes receivable, net	(641)	(86)
Due from Affiliate, Current	(424)
Device and accessory inventory	0	0
Prepaid expenses and other current assets	0	0
Total current assets	(1,065)	(86)
Investments in Subsidiaries	(62,470)	(42,654)
Property, plant and equipment, net	0	0
Due from Consolidated Affiliate	(13,777)	(13,057)
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	0	0
FCC licenses and other	0	0
Definite-lived intangible assets, net	0	0
Other assets	0	0
Total assets	(110,210)	(66,766)
Current Liabilities
Accounts Payable, Current	0	0
Accrued expenses and other current liabilities	(641)	(86)
Current portion of long-term debt, financing and capital lease obligations	0	0
Due to Affiliate, Current	(424)
Total current liabilities	(1,065)	(86)
Long-term debt, financing and capital lease obligations	0	0
Deferred tax liabilities	0	0
Note due to consolidated affiliate	(32,898)	(10,969)
Other liabilities	0	0
Due to affiliate	(13,777)	(13,057)
Total liabilities	(47,740)	(24,112)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(62,470)
Stockholders' Equity Attributable to Noncontrolling Interest	0
Total stockholders' equity	(62,470)	(42,654)
Liabilities and Equity	(110,210)	(66,766)
Due from Affiliate, Noncurrent	(32,898)	(10,969)
Parent Company [Member]
Current Assets
Cash and Cash Equivalents	0	0		0	0
Short-term Investments	0	0
Accounts and notes receivable, net	196	86
Due from Affiliate, Current	0
Device and accessory inventory	0	0
Prepaid expenses and other current assets	1	0
Total current assets	197	86
Investments in Subsidiaries	26,233	18,800
Property, plant and equipment, net	0	0
Due from Consolidated Affiliate	1	25
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	0	0
FCC licenses and other	0	0
Definite-lived intangible assets, net	0	0
Other assets	0	0
Total assets	36,838	29,305
Current Liabilities
Accounts Payable, Current	0	0
Accrued expenses and other current liabilities	196	103
Current portion of long-term debt, financing and capital lease obligations	0	0
Due to Affiliate, Current	0
Total current liabilities	196	103
Long-term debt, financing and capital lease obligations	10,407	10,394
Deferred tax liabilities	0	0
Note due to consolidated affiliate	0	0
Other liabilities	0	0
Due to affiliate	0	0
Total liabilities	10,603	10,497
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	26,235
Stockholders' Equity Attributable to Noncontrolling Interest	0
Total stockholders' equity	26,235	18,808
Liabilities and Equity	36,838	29,305
Due from Affiliate, Noncurrent	10,407	10,394
Guarantor Subsidiaries [Member]
Current Assets
Cash and Cash Equivalents	3,948	2,461		3,117	2,154
Short-term Investments	173	5,444
Accounts and notes receivable, net	445	1
Due from Affiliate, Current	424
Device and accessory inventory	0	0
Prepaid expenses and other current assets	8	11
Total current assets	4,998	7,917
Investments in Subsidiaries	36,237	23,854
Property, plant and equipment, net	0	0
Due from Consolidated Affiliate	0	13,032
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	0	0
FCC licenses and other	0	0
Definite-lived intangible assets, net	0	0
Other assets	196	134
Total assets	63,922	45,512
Current Liabilities
Accounts Payable, Current	0	0
Accrued expenses and other current liabilities	383	478
Current portion of long-term debt, financing and capital lease obligations	1,895	1,356
Due to Affiliate, Current	0
Total current liabilities	2,278	1,834
Long-term debt, financing and capital lease obligations	10,407	13,647
Deferred tax liabilities	0	0
Note due to consolidated affiliate	10,407	10,394
Other liabilities	820	837
Due to affiliate	13,777	0
Total liabilities	37,689	26,712
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	26,233
Stockholders' Equity Attributable to Noncontrolling Interest	0
Total stockholders' equity	26,233	18,800
Liabilities and Equity	63,922	45,512
Due from Affiliate, Noncurrent	22,491	575
Non-Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Other Additional Capital	4,700		$ 563
Current Assets
Cash and Cash Equivalents	492	409		$ 590	$ 487
Short-term Investments	0	0
Accounts and notes receivable, net	3,917	4,137
Due from Affiliate, Current	0
Device and accessory inventory	1,009	1,064
Prepaid expenses and other current assets	617	590
Total current assets	6,035	6,200
Investments in Subsidiaries	0	0
Property, plant and equipment, net	19,712	19,209
Due from Consolidated Affiliate	13,776	0
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	6,586	6,579
FCC licenses and other	41,222	40,585
Definite-lived intangible assets, net	2,667	3,320
Other assets	871	1,179
Total assets	90,869	77,072
Current Liabilities
Accounts Payable, Current	3,176	3,281
Accrued expenses and other current liabilities	3,921	3,646
Current portion of long-term debt, financing and capital lease obligations	2,141	3,680
Due to Affiliate, Current	424
Total current liabilities	9,662	10,607
Long-term debt, financing and capital lease obligations	12,011	11,837
Deferred tax liabilities	7,709	14,416
Note due to consolidated affiliate	22,491	575
Other liabilities	2,689	2,726
Due to affiliate	0	13,057
Total liabilities	54,562	53,218
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	36,307
Stockholders' Equity Attributable to Noncontrolling Interest	70
Total stockholders' equity	36,237	23,854
Liabilities and Equity	90,869	77,072
Due from Affiliate, Noncurrent	$ 0	$ 0