Credit Losses on Financial Instruments (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance at January 1, 2020	$ 191
Provision for expected losses	27
Write-offs charged against the allowance	(67)
Recoveries collected	11
Ending balance at March 31, 2021	162
Business
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance at January 1, 2020	109
Provision for expected losses	13
Write-offs charged against the allowance	(19)
Recoveries collected	5
Ending balance at March 31, 2021	108
Mass Markets
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance at January 1, 2020	82
Provision for expected losses	14
Write-offs charged against the allowance	(48)
Recoveries collected	6
Ending balance at March 31, 2021	54
Consumer Reclassed to Mass Markets
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance at January 1, 2020	70
Business Reclassed to Mass Markets
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance at January 1, 2020	$ 12