Shareholders' Equity (Preferred Stock) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Shareholders Equity [Line Items]
Preferred stock, liquidation amount	$ 2,675
Preferred stock	2,603	2,603
Series D Preferred Stock [Member]
Schedule Of Shareholders Equity [Line Items]
Preferred stock, issuance date	May 01, 2012
Preferred stock, earliest redemption date	May 01, 2017
Preferred stock, liquidation amount	575
Preferred stock	559
Preferred stock, dividend rate, percentage	5.85%
Series E Preferred Stock [Member]
Schedule Of Shareholders Equity [Line Items]
Preferred stock, issuance date	Jul. 31, 2012
Preferred stock, earliest redemption date	Aug. 01, 2017
Preferred stock, liquidation amount	1,150
Preferred stock	1,120
Preferred stock, dividend rate, percentage	5.625%
Series F Preferred Stock [Member]
Schedule Of Shareholders Equity [Line Items]
Preferred stock, issuance date	Oct. 31, 2012
Preferred stock, earliest redemption date	Nov. 01, 2017
Preferred stock, liquidation amount	450
Preferred stock	437
Preferred stock, dividend rate, percentage	5.20%
Series G Preferred Stock [Member]
Schedule Of Shareholders Equity [Line Items]
Preferred stock, issuance date	May 01, 2013
Preferred stock, earliest redemption date	Jun. 01, 2018
Preferred stock, liquidation amount	500
Preferred stock	$ 487
Preferred stock, dividend rate, percentage	5.20%