Retirement Benefits Retirement Benefits - Components of Accumulated Other Comprehensive Income and Regulatory Assets expected to be Amortized as Components of Periodic Benefit Cost (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Amortization of Prior Service Cost (Credit), Next Fiscal Year	$ 2
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year	640
Benefits included in accumulated other comprehensive income	642	[1]
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, after Tax	317
Pension Plan, Defined Benefit [Member] | Regulatory Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, after Tax	325
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Expected Amortization of Prior Service Cost (Credit), Next Fiscal Year	(186)
Defined Benefit Plan, Expected Amortization of Gain (Loss), Next Fiscal Year	66
Benefits included in accumulated other comprehensive income	(120)	[1]
Benefits included in regulatory assets	(55)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, after Tax	$ (65)

[1] Of the $642 million related to pension benefits at December 31, 2017, $317 million and $325 million are expected to be amortized from AOCI and regulatory assets in 2018, respectively. Of the $(120) million related to other postretirement benefits at December 31, 2017, $(65) million and $(55) million are expected to be amortized from AOCI and regulatory assets (liabilities) in 2018, respectively.