Short-Term Borrowings-Schedule (Details) - USD ($) $ in Millions		3 Months Ended															12 Months Ended
		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, 2015		Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Short-term Debt [Line Items]
Short-term borrowings		$ 4,650					$ 5,101					$ 4,995					$ 4,650		$ 5,101		$ 4,995
Securities sold under repurchase agreements [Member]
Short-term Debt [Line Items]
Short-term borrowings		$ 3,365	[1]	$ 6,494	$ 7,558	$ 4,069	$ 3,672	[1]	$ 3,986	$ 3,969	$ 5,529	$ 2,986	[1]	$ 5,870	$ 7,689	$ 12,217	$ 3,365	[1]	$ 3,672	[1]	$ 2,986	[1]
Weighted average interest rate, point in time (percent)	[1]	2.10%					1.17%					0.48%					2.10%		1.17%		0.48%
Average during year		$ 5,457		$ 12,566	$ 9,272	$ 5,050	$ 4,593		$ 5,959	$ 10,461	$ 8,117	$ 7,513		$ 6,914	$ 12,761	$ 12,251	$ 8,100		$ 7,272		$ 9,839
Weighted average interest rate, during the year (percent)																	1.28%		0.72%		0.34%
Maximum month-end balance																	$ 11,351		$ 11,950		$ 14,582
Commercial paper [Member]
Short-term Debt [Line Items]
Short-term borrowings		$ 1,154					$ 1,251					$ 1,978					$ 1,154		$ 1,251		$ 1,978
Weighted average interest rate, point in time (percent)		1.56%					1.20%					0.46%					1.56%		1.20%		0.46%
Average during year																	$ 1,256		$ 2,551		$ 3,408
Weighted average interest rate, during the year (percent)																	1.37%		0.82%		0.30%
Maximum month-end balance																	$ 1,377		$ 3,687		$ 4,870
Other [Member]
Short-term Debt [Line Items]
Short-term borrowings		$ 131					$ 178					$ 31					$ 131		$ 178		$ 31

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