Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Preferred stock outstanding	$ 1,265	$ 1,265
Preferred stock, shares outstanding (shares)	1,265	1,265
Preferred stock, par value (usd per share)	$ 0
Payments for repurchase of redeemable preferred stock	$ 0	$ 1,265	$ 300
6.0% Non-Cumulative Preferred Stock, Series I ($1,000,000 stated value)
Class of Stock [Line Items]
Dividend Rate	6.00%	6.00%
Preferred stock, cash consideration		$ 1,265
Preferred stock, redemption price per share (usd per share)	$ 1,000,000
Floating Rate Non-Cumulative Preferred Stock, Series F ($25 stated value) [Member]
Class of Stock [Line Items]
Preferred stock, par value (usd per share)		$ 25
Floating Rate Non-Cumulative Preferred Stock, Series G ($1,000 stated value) [Member]
Class of Stock [Line Items]
Preferred stock, par value (usd per share)		$ 1,000
6.50% Non-Cumulative Preferred Stock, Series H ($1,000 stated value) [Member]
Class of Stock [Line Items]
Dividend Rate		6.50%
Preferred stock, par value (usd per share)		$ 1,000