Equity (Preferred Stock - Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Class of Stock [Line Items]
Preferred Stock, Shares Issued	26,032,200		25,500,000
Preferred Stock, Par or Stated Value Per Share	$ 0.01		$ 0.01
Preferred Stock, Liquidation Preference Per Share	$ 3,405		$ 2,100
Proceeds from Issuance of Preferred Stock and Preference Stock	$ 1,274	$ 0
Series E Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued	32,200		0
Preferred Stock, Dividend Rate, Percentage	5.625%
Preferred Stock, Par or Stated Value Per Share	$ 0.01
Preferred Stock, Liquidation Preference Per Share	$ 25,000
Proceeds from Issuance of Preferred Stock and Preference Stock	$ 780
Payments of Stock Issuance Costs	$ 25
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 E Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Liquidation Preference Per Share	$ 25,500
RatingAgency [Member] | Series D Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Liquidation Preference Per Share	1,020
Depositary Shares [Member] | Series E Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Liquidation Preference Per Share	$ 25.50