Retirement Benefits - Components of Accumulated Other Comprehensive Income and Regulatory Assets expected to be Amortized as Components of Periodic Benefit Cost (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan Amounts That Will Be Amortized From Accumulated Other Comprehensive Income Loss And Regulatory Assets In Next Fiscal Year	$ (101)
Benefits included in accumulated other comprehensive income	(51)
Benefits included in regulatory assets	(50)
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Future Amortization of Prior Service Cost (Credit)	13
Defined Benefit Plan, Future Amortization of Gain (Loss)	562
Benefits included in accumulated other comprehensive income	575	[1]
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Net of Tax	329
Pension Plan, Defined Benefit [Member] | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Net of Tax	246
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Gross Prescription Drug Subsidy Receipts Received			10
Defined Benefit Plan, Future Amortization of Prior Service Cost (Credit)	(175)
Defined Benefit Plan, Future Amortization of Gain (Loss)	74
Benefits included in accumulated other comprehensive income	$ (101)	[1]

[1] )Of the $575 million related to pension benefits at December 31, 2014, $329 million and $246 million are expected to be amortized from AOCI and regulatory assets in 2015, respectively. Of the $101 million related to other postretirement benefits at December 31, 2014, $(51) million and $(50) million are expected to be amortized from AOCI and regulatory assets (liabilities) in 2015, respectivel