Retirement Benefits- Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Increase (Decrease) in Obligation, Pension Benefits	$ 75
Benefit obligation increase (decrease) reflecting actual census data	36
Accumulated Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), after Tax	39
Regulatory asset increase (decrease) due to updated valuation adjustment	53
Defined benefit plan, amounts recognized In regulatory liabilities, before tax	$ 5
Defined Contribution Plan, Employer Discretionary Contribution Amount		$ 161	$ 179	$ 128
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount		0
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset increase (decrease) due to updated valuation adjustment		$ 3	$ (6)	$ (423)
Expected return on plan assets		7.00%	7.00%	7.00%
Mortality Rate		0.75%	0.75%
Discount rate		4.31%	3.62%	4.04%
Defined Contribution Plan, Employer Discretionary Contribution Amount		$ 500
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Regulatory asset increase (decrease) due to updated valuation adjustment		$ 106	$ (57)	$ 125
Expected return on plan assets		6.67%	6.60%	6.58%
Mortality Rate		0.75%	0.75%
Discount rate		4.30%	3.61%	4.04%
Change in Assumptions for Defined Benefit Plans [Member] | Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets		7.00%
Discount rate		4.31%
Change in Assumptions for Defined Benefit Plans [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets		6.67%
Discount rate		4.30%
Assets [Member] | Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate		18.80%
Assets [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Discount rate		14.40%
Subsequent Event [Member] | Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets		7.00%
Subsequent Event [Member] | Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected return on plan assets		6.69%