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, 2016 USD ($)
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Future Amortization of Prior Service Cost (Credit)	$ 1
Defined Benefit Plan, Future Amortization of Gain (Loss)	605
Benefits included in accumulated other comprehensive income	606	[1]
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Net of Tax	297
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	309
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Future Amortization of Prior Service Cost (Credit)	(188)
Defined Benefit Plan, Future Amortization of Gain (Loss)	55
Benefits included in accumulated other comprehensive income	(133)	[1]
Benefits included in regulatory assets	(63)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI, Pension and Other Postretirement Benefit Plans, Net of Tax	$ (70)

[1] ___________________ (a)Of the $606 million related to pension benefits at December 31, 2016, $297 million and $309 million are expected to be amortized from AOCI and regulatory assets in 2017, respectively. Of the $(133) million related to other postretirement benefits at December 31, 2016, $(70) million and $(63) million are expected to be amortized from AOCI and regulatory assets (liabilities) in 2017, respectively.