Allowance for Losses on Financial Assets Allowance for Losses on Financial Assets (Allowance Disclosure) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Jan. 01, 2020
Allowance for Losses on Financial Assets [Line Items]
Allowance for Credit Loss	$ (793)	$ (711)	$ (802)	$ (337)
Credit Loss Expense, Reversal	(27)	(377)
Allowance for Credit Loss, Writeoff	35	3
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Accounts Receivable, Allowance for Credit Loss	(407)	(431)	(444)	(138)
Accounts Receivable, Credit Loss Expense (Reversal)	15	296
Accounts Receivable, Allowance for Credit Loss, Writeoff	21	3
Credit Loss, Recoveries	1
Accounts Receivable, Allowance for Credit Loss, Recovery	1
Contract with Customer, Asset, Allowance for Credit Loss [Roll Forward]
Contract with Customer, Asset, Allowance for Credit Loss	(126)	(127)	(129)	(81)
Contract with Customer, Asset, Credit Loss Expense (Reversal)	3	46
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	(14)	(17)	(17)	(5)
Financing Receivable, Credit Loss, Expense (Reversal)	3	12
Other Current Assets
Allowance for Losses on Financial Assets [Line Items]
Allowance for Credit Loss	(74)	(58)	(72)	(38)
Credit Loss Expense, Reversal	(3)	(20)
Allowance for Credit Loss, Writeoff	1
Other Noncurrent Assets
Allowance for Losses on Financial Assets [Line Items]
Allowance for Credit Loss	(172)	(78)	$ (140)	$ (75)
Credit Loss Expense, Reversal	(45)	$ (3)
Allowance for Credit Loss, Writeoff	$ 13