Guarantor Financial Information (CONDENSED CONSOLIDATING BALANCE SHEET) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016
Current Assets
Cash and Cash Equivalents	$ 4,802	$ 2,870	$ 4,006	$ 4,006	$ 2,641
Short-term Investments	1,610	5,444
Accounts and notes receivable, net	4,118	4,138
Due from Affiliate, Current	0
Device and accessory inventory	751	1,064
Prepaid expenses and other current assets	654	601
Total current assets	11,935	14,117
Investments in Subsidiaries	0	0
Property, plant and equipment, net	18,901	19,209
Due from Consolidated Affiliate	0	0
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	6,578	6,579
FCC licenses and other	41,072	40,585
Definite-lived intangible assets, net	2,848	3,320
Other assets	1,132	1,313
Total assets	82,466	85,123
Current Liabilities
Accounts Payable, Current	2,947	3,281
Accrued expenses and other current liabilities	3,808	4,141
Current portion of long-term debt, financing and capital lease obligations	4,142	5,036
Due to Affiliate, Current	0
Total current liabilities	10,897	12,458
Long-term debt, financing and capital lease obligations	34,236	35,878
Deferred tax liabilities	14,780	14,416
Note due to consolidated affiliate	0	0
Other liabilities	3,533	3,563
Due to affiliate	0	0
Total liabilities	63,446	66,315
Commitments and contingencies
Total stockholders' equity	19,020	18,808
Liabilities and Equity	82,466	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	(323)	(86)
Due from Affiliate, Current	(424)
Device and accessory inventory	0	0
Prepaid expenses and other current assets	0	0
Total current assets	(747)	(86)
Investments in Subsidiaries	(43,696)	(42,654)
Property, plant and equipment, net	0	0
Due from Consolidated Affiliate	(213)	(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	(66,314)	(66,766)
Current Liabilities
Accounts Payable, Current	0	0
Accrued expenses and other current liabilities	(323)	(86)
Current portion of long-term debt, financing and capital lease obligations	0	0
Due to Affiliate, Current	(424)
Total current liabilities	(747)	(86)
Long-term debt, financing and capital lease obligations	0	0
Deferred tax liabilities	0	0
Note due to consolidated affiliate	(21,658)	(10,969)
Other liabilities	0	0
Due to affiliate	(213)	(13,057)
Total liabilities	(22,618)	(24,112)
Total stockholders' equity	(43,696)	(42,654)
Liabilities and Equity	(66,314)	(66,766)
Due from Affiliate, Noncurrent	(21,658)	(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	87	86
Due from Affiliate, Current	0
Device and accessory inventory	0	0
Prepaid expenses and other current assets	0	0
Total current assets	87	86
Investments in Subsidiaries	19,012	18,800
Property, plant and equipment, net	0	0
Due from Consolidated Affiliate	9	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	29,510	29,305
Current Liabilities
Accounts Payable, Current	0	0
Accrued expenses and other current liabilities	88	103
Current portion of long-term debt, financing and capital lease obligations	0	0
Due to Affiliate, Current	0
Total current liabilities	88	103
Long-term debt, financing and capital lease obligations	10,402	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	10,490	10,497
Total stockholders' equity	19,020	18,808
Liabilities and Equity	29,510	29,305
Due from Affiliate, Noncurrent	10,402	10,394
Guarantor Subsidiaries [Member]
Current Assets
Cash and Cash Equivalents	4,319	2,461		3,509	2,154
Short-term Investments	1,610	5,444
Accounts and notes receivable, net	236	1
Due from Affiliate, Current	424
Device and accessory inventory	0	0
Prepaid expenses and other current assets	12	11
Total current assets	6,601	7,917
Investments in Subsidiaries	24,684	23,854
Property, plant and equipment, net	0	0
Due from Consolidated Affiliate	204	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	192	134
Total assets	42,937	45,512
Current Liabilities
Accounts Payable, Current	0	0
Accrued expenses and other current liabilities	338	478
Current portion of long-term debt, financing and capital lease obligations	40	1,356
Due to Affiliate, Current	0
Total current liabilities	378	1,834
Long-term debt, financing and capital lease obligations	12,303	13,647
Deferred tax liabilities	0	0
Note due to consolidated affiliate	10,402	10,394
Other liabilities	842	837
Due to affiliate	0	0
Total liabilities	23,925	26,712
Total stockholders' equity	19,012	18,800
Liabilities and Equity	42,937	45,512
Due from Affiliate, Noncurrent	11,256	575
Non-Guarantor Subsidiaries [Member]
Condensed Balance Sheet Statements, Captions [Line Items]
Other Additional Capital	231		$ 563
Current Assets
Cash and Cash Equivalents	483	409		$ 497	$ 487
Short-term Investments	0	0
Accounts and notes receivable, net	4,118	4,137
Due from Affiliate, Current	0
Device and accessory inventory	751	1,064
Prepaid expenses and other current assets	642	590
Total current assets	5,994	6,200
Investments in Subsidiaries	0	0
Property, plant and equipment, net	18,901	19,209
Due from Consolidated Affiliate	0	0
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	6,578	6,579
FCC licenses and other	41,072	40,585
Definite-lived intangible assets, net	2,848	3,320
Other assets	940	1,179
Total assets	76,333	77,072
Current Liabilities
Accounts Payable, Current	2,947	3,281
Accrued expenses and other current liabilities	3,705	3,646
Current portion of long-term debt, financing and capital lease obligations	4,102	3,680
Due to Affiliate, Current	424
Total current liabilities	11,178	10,607
Long-term debt, financing and capital lease obligations	11,531	11,837
Deferred tax liabilities	14,780	14,416
Note due to consolidated affiliate	11,256	575
Other liabilities	2,691	2,726
Due to affiliate	213	13,057
Total liabilities	51,649	53,218
Total stockholders' equity	24,684	23,854
Liabilities and Equity	76,333	77,072
Due from Affiliate, Noncurrent	$ 0	$ 0