Allowance for Losses on Financial Assets Allowance for Losses on Financial Assets (Allowance Disclosure) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Jan. 01, 2020
Allowance for Losses on Financial Assets [Line Items]
Allowance for Credit Loss	$ (840)	$ (374)	$ (802)	$ (337)
Credit Loss Expense, Reversal	(39)	(38)
Allowance for Credit Loss, Writeoff	1	1
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Accounts Receivable, Allowance for Credit Loss	(433)	(166)	(444)	(138)
Accounts Receivable, Credit Loss Expense (Reversal)	10	29
Accounts Receivable, Allowance for Credit Loss, Writeoff	1	1
Contract with Customer, Asset, Allowance for Credit Loss [Roll Forward]
Contract with Customer, Asset, Allowance for Credit Loss	(130)	(80)	(129)	(81)
Contract with Customer, Asset, Credit Loss Expense (Reversal)	(1)	1
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Financing Receivable, Allowance for Credit Loss	(17)	(5)	(17)	(5)
Other Current Assets
Allowance for Losses on Financial Assets [Line Items]
Allowance for Credit Loss	(78)	(48)	(72)	(38)
Credit Loss Expense, Reversal	(6)	(10)
Other Noncurrent Assets
Allowance for Losses on Financial Assets [Line Items]
Allowance for Credit Loss	(182)	$ (75)	$ (140)	$ (75)
Credit Loss Expense, Reversal	$ (42)