Capital - Additional Information (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		6 Months Ended	9 Months Ended	12 Months Ended
		Jun. 30, 2018	Sep. 30, 2018	Dec. 31, 2018		Dec. 31, 2017	Dec. 31, 2016
Schedule of Capitalization, Equity [Line Items]
Common Stock, Dividends, Per Share, Cash Paid		$ 0.40	$ 0.50	$ 1.80		$ 1.32	$ 1.00
Dividends, Common Stock, Cash				$ 826		$ 634	$ 498
Dividends, Preferred Stock, Cash	[1]			107		94	$ 66
Cash Reserve Deposit Required and Made				1,300		$ 1,200
Common Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Stock Repurchased During Period, Value		$ 660		$ 1,250
Stock Repurchased During Period, Shares				18.9
Stock Repurchase Program, Remaining Authorized Repurchase Amount				$ 750
Series A Preferred Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid				$ 4,056		$ 4,056	$ 4,067
Series B Preferred Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid				4,056		4,056	4,067
Series E Preferred Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid				1,469	[2]	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	5,625
Series G Preferred Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid				5,050		3,128	0
Series H Preferred Stock [Member]
Schedule of Capitalization, Equity [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid				$ 5,566		$ 669	$ 0
Parent Company [Member]
Schedule of Capitalization, Equity [Line Items]
Retained Earnings, Unappropriated				$ 2,200		$ 2,500
Retained Earnings [Member]
Schedule of Capitalization, Equity [Line Items]
Dividends, Common Stock, Cash				826		634	$ 498
Dividends, Preferred Stock, Cash	[1]			$ 107		$ 94	$ 66

[1] For the year ended December 31, 2018, dividends were $4,056 per share for both Series A and B Preferred Stock, $1,469 per share for Series E Preferred Stock, $5,625 per share for Series F Preferred Stock, $5,050 per share for Series G Preferred Stock, and $5,566 per share for Series H Preferred Stock. For the year ended December 31, 2017, dividends were $4,056 per share for both Series A and B Preferred Stock, $5,875 per share for Series E Preferred Stock, $5,625 per share for Series F Preferred Stock, $3,128 per share for Series G Preferred Stock, and $669 per share for Series H Preferred Stock. For the year ended December 31, 2016, dividends were $4,067 per share for both Series A and B Preferred Stock, $5,875 per share for Series E Preferred Stock, and $5,625 per share for Series F Preferred Stock.

[2] All 4,500 shares of outstanding Series E Preferred Stock were redeemed in the first quarter of 2018.