Short-Term Borrowings-Schedule (Details) - USD ($) $ in Millions		3 Months Ended															12 Months Ended
		Dec. 31, 2020		Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019		Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Short-term Debt [Line Items]
Short-term borrowings		$ 4,952					$ 3,659					$ 4,180					$ 4,952		$ 3,659		$ 4,180
Securities sold under repurchase agreements [Member]
Short-term Debt [Line Items]
Short-term borrowings		$ 1,765	[1]	$ 2,185	$ 2,218	$ 12,497	$ 1,072	[1]	$ 5,568	$ 3,553	$ 3,714	$ 1,466	[1]	$ 2,471	$ 2,743	$ 2,792	$ 1,765	[1]	$ 1,072	[1]	$ 1,466	[1]
Weighted average interest rate, point in time (percent)	[1]	0.22%					1.75%					4.16%					0.22%		1.75%		4.16%
Average during year		$ 2,140		$ 2,534	$ 7,210	$ 7,240	$ 1,483		$ 8,432	$ 3,433	$ 3,142	$ 3,045		$ 3,550	$ 5,008	$ 5,182	$ 4,768		$ 4,130		$ 4,189
Weighted average interest rate, during the period (percent)																	0.70%		3.28%		2.86%
Maximum month-end balance																	$ 12,497		$ 8,569		$ 5,829
Commercial paper [Member]
Short-term Debt [Line Items]
Short-term borrowings		$ 2,998					$ 2,476					$ 2,532					$ 2,998		$ 2,476		$ 2,532
Weighted average interest rate, point in time (percent)		0.52%					2.21%					2.70%					0.52%		2.21%		2.70%
Other [Member]
Short-term Debt [Line Items]
Short-term borrowings		$ 189					$ 111					$ 182					$ 189		$ 111		$ 182

[1] The following table presents the quarter end and average quarterly balances of securities sold under repurchase agreements: