Short-Term Borrowings-Schedule (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Short-term borrowings	$ 4,180				$ 4,650				$ 5,101				$ 4,180	$ 4,650	$ 5,101
Securities sold under repurchase agreements [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 1,466	$ 2,471	$ 2,743	$ 2,792	$ 3,365	$ 6,494	$ 7,558	$ 4,069	$ 3,672	$ 3,986	$ 3,969	$ 5,529	$ 1,466	$ 3,365	$ 3,672
Weighted average interest rate, point in time (percent)	4.16%				2.10%				1.17%				4.16%	2.10%	1.17%
Average during year	$ 3,045	$ 3,550	$ 5,008	$ 5,182	$ 5,457	$ 12,566	$ 9,272	$ 5,050	$ 4,593	$ 5,959	$ 10,461	$ 8,117	$ 4,189	$ 8,100	$ 7,272
Weighted average interest rate, during the year (percent)													2.86%	1.28%	0.72%
Maximum month-end balance													$ 5,829	$ 11,351	$ 11,950
Commercial paper [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 2,532				$ 1,154				$ 1,251				$ 2,532	$ 1,154	$ 1,251
Weighted average interest rate, point in time (percent)	2.70%				1.56%				1.20%				2.70%	1.56%	1.20%
Other [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 182				$ 131				$ 178				$ 182	$ 131	$ 178