Equity (Preferred Stock - Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Jul. 01, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 12, 2015	Jun. 01, 2015
Class of Stock [Line Items]
Preferred Stock, Shares Issued		25,500,000	25,500,000
Preferred stock issued, net of issuance costs		$ 0	$ 0	$ 1,483
Payments of Stock Issuance Costs				$ 17
Tender Offer Number Of Shares					59,850,000
Stock Redeemed or Called During Period, Shares	22,807,587			37,192,413
Preferred Stock, Redemption Amount	$ 570			$ 932
Preferred stock repurchase premium		$ 0	$ 0	$ 42
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock, shares outstanding		25,500,000	25,500,000
Preferred stock, par value		$ 0.01	$ 0.01
Preferred stock, aggregate liquidation preference		$ 2,100	$ 2,100
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued		1,500,000	1,500,000			1,500,000
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock, shares outstanding		1,500,000	1,500,000
Preferred Stock, Dividend Payment Rate, Variable				5.25% Fixed-to-Floating Rate
Preferred stock, par value						$ 0.01
Preferred stock, aggregate liquidation preference						$ 1,000
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued		24,000,000	24,000,000
Preferred stock redemption price per share		$ 25
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock, shares outstanding		24,000,000	24,000,000
Preferred Stock, Dividend Payment Rate, Variable		(i) 1.00% above three-month LIBOR on the related LIBOR determination date; or (ii) 4.00%
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage				6.50%
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred stock, shares outstanding					60,000,000
Preferred stock, par value				$ 0.01
Preferred stock, aggregate liquidation preference					$ 25
Fixed Rate [Member] | Series C Preferred Stock [Member]
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock, Dividend Payment Rate, Variable		0.0525
Variable Rate [Member] | Series C Preferred Stock [Member]
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock, Dividend Payment Rate, Variable		three-month LIBOR plus 3.575%