Severance and Exit Costs (Schedule of Severance and Exit Costs) (Details) - USD ($) $ in Millions	6 Months Ended
	Sep. 30, 2017	Mar. 31, 2017
Restructuring Cost and Reserve [Line Items]
Document Period End Date	Sep. 30, 2017
Restructuring Reserve [Roll Forward]
Restructuring Reserve	$ 224	$ 301
Supplemental Unemployment Benefits, Severance Benefits	8	12
Restructuring Charges	0
Severance Costs	5
Payments for Postemployment Benefits	(9)
Restructuring Reserve, Settled with Cash	(77)
Lease [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve	188	249
Restructuring reserved settled with cash and other adjustments	(52)
Access [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve	28	$ 40
Restructuring reserved settled with cash and other adjustments	(16)
Wireless [Member]
Restructuring Reserve [Roll Forward]
Severance Costs	3
Wireless [Member] | Lease [Member]
Restructuring Reserve [Roll Forward]
Restructuring Charges	(9)
Wireline [Member]
Restructuring Reserve [Roll Forward]
Severance Costs	2
Severance and exit costs [Member] | Access [Member]
Restructuring Reserve [Roll Forward]
Restructuring Charges	4
Severance and exit costs [Member] | Wireless [Member] | Access [Member]
Restructuring Reserve [Roll Forward]
Restructuring Charges	1
Severance and exit costs [Member] | Wireline [Member] | Access [Member]
Restructuring Reserve [Roll Forward]
Restructuring Charges	$ 3