Equity (Preferred Stock - Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Nov. 01, 2017
Class of Stock [Line Items]
Stock Repurchase Program, Authorized Amount				$ 2,000
Preferred Stock, Shares Issued	26,000,000		25,500,000
Preferred Stock, Par or Stated Value Per Share	$ 0.01		$ 0.01
Preferred Stock, Liquidation Preference Per Share	$ 2,600		$ 2,100
Proceeds from Issuance of Preferred Stock and Preference Stock	$ 494	$ 0
Series D Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued	500,000		0
Preferred Stock, Dividend Payment Rate, Variable	5.875% Fixed-to-Floating Rate
Preferred Stock, Par or Stated Value Per Share	$ 0.01
Preferred Stock, Liquidation Preference Per Share	$ 1,000
Proceeds from Issuance of Preferred Stock and Preference Stock	$ 494
Payments of Stock Issuance Costs	$ 6
Fixed Rate [Member] | Series D Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Payment Rate, Variable	0.05875
Variable Rate [Member] | Series D Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Payment Rate, Variable	LIBOR plus 2.959%
RatingAgency [Member] | Series D Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Liquidation Preference Per Share	$ 1,020