Short-Term Borrowings-Schedule (Details) - USD ($) $ in Millions		3 Months Ended															12 Months Ended
		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, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Short-term Debt [Line Items]
Short-term borrowings		$ 5,101					$ 4,995					$ 12,795					$ 5,101		$ 4,995		$ 12,795
Securities sold under repurchase agreements [Member]
Short-term Debt [Line Items]
Short-term borrowings		3,672	[1]	$ 3,986	$ 3,969	$ 5,529	2,986	[1]	$ 5,870	$ 7,689	$ 12,217	7,707	[1]	$ 6,634	$ 6,481	$ 19,254	3,672	[1]	2,986	[1]	7,707	[1]
Average during year		$ 4,593		$ 5,959	$ 10,461	$ 8,117	$ 7,513		$ 6,914	$ 12,761	$ 12,251	$ 8,902		$ 9,106	$ 16,849	$ 19,300	7,272		9,839		13,498
Maximum month-end balance																	$ 11,950		$ 14,582		$ 19,254
Interest rate at the end of the period	[1]	1.17%					0.48%					0.32%					1.17%		0.48%		0.32%
Interest rate average during year		0.72%					0.34%					0.16%					0.72%		0.34%		0.16%
Commercial paper [Member]
Short-term Debt [Line Items]
Short-term borrowings		$ 1,251					$ 1,978					$ 4,772					$ 1,251		$ 1,978		$ 4,772
Average during year																	2,551		3,408		4,537
Maximum month-end balance																	$ 3,687		$ 4,870		$ 4,963
Interest rate at the end of the period		1.20%					0.46%					0.23%					1.20%		0.46%		0.23%
Interest rate average during year		0.82%					0.30%					0.23%					0.82%		0.30%		0.23%
Other [Member]
Short-term Debt [Line Items]
Short-term borrowings		$ 178					$ 31					$ 316					$ 178		$ 31		$ 316

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