Employee Benefit Plans Employee Benefit Plans - Unrecognized Amounts (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of Amounts Not Yet Recognized As Components of Net Periodic Benefit Cost [Roll Forward]
Amortization of net loss	$ 72
Amoritization of prior service credit	(15)
Amortization of regulatory deferrals	0
Amounts that will be amortized in next fiscal year	57
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Net loss	660	630
Prior service credit	(37)	(45)
Regulatory deferrals	(5)	(7)
Total net periodic benefit cost not yet recognized	618	578	441
Reconciliation of Amounts Not Yet Recognized As Components of Net Periodic Benefit Cost [Roll Forward]
Net loss arising during the year	74	161
Prior service credit arising during the year		(4)
Net amortization	(34)	(20)
Total Net Periodic Benefit Cost Not Yet Recognized, Arising During Period	40	137
Amortization of net loss	57
Amoritization of prior service credit	(8)
Amortization of regulatory deferrals	(1)
Amounts that will be amortized in next fiscal year	48
Other Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Net loss	214	206
Prior service credit	(40)	(46)
Regulatory deferrals	3	3
Total net periodic benefit cost not yet recognized	177	163
Reconciliation of Amounts Not Yet Recognized As Components of Net Periodic Benefit Cost [Roll Forward]
Amortization of net loss	15
Amoritization of prior service credit	(7)
Amortization of regulatory deferrals	1
Amounts that will be amortized in next fiscal year	9
Regulatory Asset [Member] | Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net periodic benefit cost not yet recognized	599	564	430
Reconciliation of Amounts Not Yet Recognized As Components of Net Periodic Benefit Cost [Roll Forward]
Net loss arising during the year	68	157
Prior service credit arising during the year		(4)
Net amortization	(33)	(19)
Total Net Periodic Benefit Cost Not Yet Recognized, Arising During Period	35	134
Regulatory Asset [Member] | Other Postretirement [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net periodic benefit cost not yet recognized	177	163	165
Reconciliation of Amounts Not Yet Recognized As Components of Net Periodic Benefit Cost [Roll Forward]
Net loss arising during the year	18	70
Prior service credit arising during the year		(46)
Reduction In net transition obligation		(8)
Net amortization	(4)	(18)
Total Net Periodic Benefit Cost Not Yet Recognized, Arising During Period	14	(2)
Accumulated Other Comprehensive Loss [Member] | Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Total net periodic benefit cost not yet recognized	19	14	11
Reconciliation of Amounts Not Yet Recognized As Components of Net Periodic Benefit Cost [Roll Forward]
Net loss arising during the year	6	4
Prior service credit arising during the year		0
Net amortization	(1)	(1)
Total Net Periodic Benefit Cost Not Yet Recognized, Arising During Period	$ 5	$ 3