Capital - Additional Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended	6 Months Ended	12 Months Ended
		Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Capitalization, Equity [Line Items]
Common Stock, Dividends, Per Share, Cash Paid		$ 0.40	$ 0.26	$ 1.32	$ 1.00	$ 0.92
Dividends, Common Stock, Cash				$ (634)	$ (498)	$ (475)
Dividends, Preferred Stock, Cash	[1]			$ (94)	(66)	$ (64)
Dividend Per Quarter Threshold Prior To Tenth Anniversay Triggering Execise Of Warrants				$ 0.54
Cash Reserve Deposit Required and Made				$ 1,200	$ 1,300
Common Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Stock Repurchased During Period, Value			$ 480	660
Stock Repurchase Program, Remaining Authorized Repurchase Amount				$ 660
Stock Repurchased During Period, Shares				11.1
Series A Preferred Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid				$ 4,056	$ 4,067	$ 4,056
Series B Preferred Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid				4,056	4,067	4,056
Series E Preferred Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid				5,875	5,875	5,875
Series F Preferred Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid				5,625	$ 5,625	$ 6,219
Series G Preferred Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid				3,128
Series H Preferred Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid				$ 669
Parent Company [Member]
Schedule of Capitalization, Equity [Line Items]
Retained Earnings, Unappropriated				$ 2,500	$ 2,500

[1] For the year ended December 31, 2017, dividends were $4,056 per share for both Perpetual Preferred Stock Series A and B, $5,875 per share for Perpetual Preferred Stock Series E, $5,625 per share for Perpetual Preferred Stock Series F, $3,128 per share for Perpetual Preferred Stock Series G, and $669 per share for Perpetual Preferred Stock Series H.For the year ended December 31, 2016, dividends were $4,067 per share for both Perpetual Preferred Stock Series A and B, $5,875 per share for Perpetual Preferred Stock Series E, and $5,625 per share for Perpetual Preferred Stock Series F.For the year ended December 31, 2015, dividends were $4,056 per share for both Perpetual Preferred Stock Series A and B, and $5,875 per share for Perpetual Preferred Stock Series E, and $6,219 per share for Perpetual Preferred Stock Series F.