Severance and Exit Costs (Schedule of Severance and Exit Costs Liability) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning	$ 248	$ 301
Severance costs, beginning	64	12
Restructuring charges	(85)	(80)
Severance costs	23	79
Restructuring reserve, settled with cash	(160)	(133)
Payments for postemployment benefits	(81)	(27)
Restructuring reserve, end	173	248
Severance costs, end	6	64
Lease [Member]
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning	165	249
Restructuring charges	(25)	2
Restructuring reserved settled with cash and other adjustments	59	82
Restructuring reserve, end	131	165
Access [Member]
Restructuring Reserve [Roll Forward]
Restructuring reserve, beginning	19	40
Restructuring reserved settled with cash and other adjustments	20	24
Restructuring reserve, end	36	19
Severance and exit costs [Member] | Access [Member]
Restructuring Reserve [Roll Forward]
Restructuring charges	(37)	(3)
Wireline [Member]
Restructuring Reserve [Roll Forward]
Severance costs	8	6
Wireline [Member] | Severance and exit costs [Member] | Access [Member]
Restructuring Reserve [Roll Forward]
Restructuring charges	(11)	(13)
Wireless [Member]
Restructuring Reserve [Roll Forward]
Severance costs	15	73
Wireless [Member] | Severance and exit costs [Member] | Access [Member]
Restructuring Reserve [Roll Forward]
Restructuring charges	$ (26)	$ 10