Severance and Exit Costs (Schedule of Severance and Exit Costs) (Details) - USD ($) $ in Millions	6 Months Ended
	Sep. 30, 2018	Mar. 31, 2018
Restructuring Reserve [Roll Forward]
Restructuring Reserve	$ 193	$ 248
Supplemental Unemployment Benefits, Severance Benefits	27	64
Restructuring Charges	33
Severance Costs	14
Payments for Postemployment Benefits	(51)
Restructuring Reserve, Settled with Cash	(88)
Lease [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve	142	165
Restructuring Charges	8
Restructuring reserved settled with cash and other adjustments	31
Access [Member]
Restructuring Reserve [Roll Forward]
Restructuring Reserve	24	$ 19
Restructuring reserved settled with cash and other adjustments	6
Wireless [Member]
Restructuring Reserve [Roll Forward]
Severance Costs	6
Wireless [Member] | Lease [Member]
Restructuring Reserve [Roll Forward]
Restructuring Charges	8
Wireline [Member]
Restructuring Reserve [Roll Forward]
Severance Costs	8
Severance and exit costs [Member] | Access [Member]
Restructuring Reserve [Roll Forward]
Restructuring Charges	11
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	$ 10