Retirement Benefits - Narrative (Details) $ in Millions	3 Months Ended
	Mar. 31, 2021 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Increase (Decrease) in Obligation, Pension Benefits	$ 33
Increase (Decrease) in Obligation, Other Postretirement Benefits	(9)
Accumulated Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Tax	1
Defined Benefit Plan Amounts Recognized In Regulatory Assets Before Tax	21
Defined Benefit Plan Amounts Recognized In Regulatory Liabilities Before Tax	$ 1
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets	7.00%
Discount rate	2.58%
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected return on assets	6.46%
Discount rate	2.51%