Severance and Exit Costs (Schedule of Severance and Exit Costs Liability) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning	$ 301	$ 525
Severance costs, beginning	12	150
Restructuring charges	(80)	(68)
Severance costs	79	20
Restructuring reserve, settled with cash	(133)	(292)
Payments for postemployment benefits	(27)	(158)
Restructuring reserve, end	248	301
Severance costs, end	64	12
Lease [Member]
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning	249	338
Restructuring charges	(2)	(17)
Restructuring reserved settled with cash and other adjustments	(82)	(106)
Restructuring reserve, end	165	249
Access [Member]
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning	40	37
Restructuring charges		(31)
Restructuring reserved settled with cash and other adjustments	(24)	(28)
Restructuring reserve, end	19	40
Severance and exit costs [Member] | Access [Member]
Restructuring Reserve [Roll Forward]
Restructuring charges	(3)	(29)
Wireline [Member]
Restructuring Reserve [Roll Forward]
Severance costs	6	1
Wireline [Member] | Lease [Member]
Restructuring Reserve [Roll Forward]
Restructuring charges	(3)	(3)
Wireline [Member] | cost of products [Member] | Access [Member]
Restructuring Reserve [Roll Forward]
Restructuring charges		(2)
Wireline [Member] | Severance and exit costs [Member] | Access [Member]
Restructuring Reserve [Roll Forward]
Restructuring charges	(13)	(17)
Wireless [Member]
Restructuring Reserve [Roll Forward]
Severance costs	73	19
Wireless [Member] | Lease [Member]
Restructuring Reserve [Roll Forward]
Restructuring charges	(5)	(14)
Wireless [Member] | Severance and exit costs [Member] | Access [Member]
Restructuring Reserve [Roll Forward]
Restructuring charges	$ (10)	$ (12)