Short-Term Borrowings-Quarterly 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]
Quarter end balance	$ 5,101					$ 4,995					$ 12,795					$ 5,101		$ 4,995		$ 12,795
Securities sold under repurchase agreements [Member]
Short-term Debt [Line Items]
Quarter end balance	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 quarterly balance	$ 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

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