Common Shareholders' Equity (Accumulated Other Comprehensive Income) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Components Of Comprehensive Income And Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (255)	$ (154)	$ 166	$ 169
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(101)	(320)	(3)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	1,810	1,603	1,954
Cash Flow Hedging [Member]
Components Of Comprehensive Income And Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(266)	(204)	24	67
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(62)	(228)	(43)
Available-for-sale Securities [Member]
Components Of Comprehensive Income And Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	96	103	133	94
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(7)	(30)	39
Defined Benefit Plans and Other Postretirement Benefit Plans [Domain]
Components Of Comprehensive Income And Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(74)	(46)	(1)	(3)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(28)	(45)	2
Accumulated Translation Adjustment [Member]
Components Of Comprehensive Income And Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	12	5	10	11
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	7	(5)	(1)
Other ComprehesiveIncomeEquityMethod [Member]
Components Of Comprehensive Income And Accumulated Other Comprehensive Income Loss [Line Items]
Accumulated Other Comprehensive Income (Loss), Net of Tax	(23)	(12)	0	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	$ (11)	$ (12)	$ 0