Sales Of Receivables (Equipment Installment and AT&T Revolving Programs) (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2020
Equipment Installment Program | Guarantee Obligation
Accounts, Notes, Loans and Financing Receivable [Line Items]
Guarantee obligation			$ 384	$ 319
Equipment Installment Program | Accounts payable and accrued liabilities | Guarantee Obligation
Accounts, Notes, Loans and Financing Receivable [Line Items]
Guarantee obligation			148	234
Equipment Installment Program | Deferred Purchase Price Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred purchase price receivable			2,336	2,163
Equipment Installment Program | Deferred Purchase Price Receivable | Other current assets
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred purchase price receivable			$ 1,569	1,538
Revolving Receivables Program
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revolving agreement, term			1 year
Revolving agreement, transfer amount (up to)		$ 5,300	$ 4,300
Revolving agreement extension term	1 year
Receivables pledged as collateral				$ 3,516