Intangible Assets (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	May 23, 2018
Acquired Finite-Lived Intangible Assets [Line Items]
Entity Common Stock, Price Per Share	$ 4.88
Net book value per share price	$ 6.58
FCC Licenses [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Fair Value, Intangible Asset	$ 921
Other, Net [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gain (Loss) on Disposition of Intangible Assets	$ 479	$ 354
Corporate Joint Venture [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Preferred Economic Interest, Ownership Percent	68.00%
Common Voting Interest	55.00%
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	$ 390
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	240
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets, Fair Value	270
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities, Fair Value	220
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current and Fixed Assets	85
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt	170
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities	50
Other Intangible Assets [Member] | Corporate Joint Venture [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	35
FCC Licenses [Member] | Corporate Joint Venture [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles	$ 150
PRWireless PR, Inc. [Member] | Corporate Joint Venture [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Preferred Economic Interest, Ownership Percent	32.00%
Common Voting Interest	45.00%
Subsequent Event [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Entity Common Stock, Price Per Share			$ 5.14