Income Taxes (Deferred Income Tax Assets and Liabilities) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Deferred tax assets
Net operating loss carryforwards, long-term	$ 4,116	$ 6,812
Tax credit carryforwards, long-term	244	340
Capital loss carryforwards, long-term	0	1
Property, plant and equipment, long-term	2,192	2,192
Debt obligations, long-term	64	205
Deferred rent, long-term	231	402
Pension and other postretirement benefits, long-term	219	332
Accruals and other liabilities, long-term	913	1,454
Deferred tax assets, gross, noncurrent	7,979	11,738
Valuation allowance, noncurrent	(4,745)	(10,477)
Deferred tax assets, net of valuation allowance, noncurrent	3,234	1,261
Deferred tax liabilities
FCC Licenses	8,877	12,876
Trademarks	1,131	1,712
Intangibles	298	771
Other, long-term	222	318
Deferred tax liabilities, long-term	10,528	15,677
Long-term deferred tax liability	$ 7,294	$ 14,416