Retirement Benefits - Components of Accumulated Other Comprehensive Income and Regulatory Assets (Liabilities) related to Retirement Plans (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, amounts recognized in regulatory assets, before tax	$ 53
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss		$ 538	$ 635	$ (222)
Amortization of actuarial loss		(414)	(629)	(607)
Current year prior service cost (credit)		68	(4)	9
Amortization of prior service (cost) credit		0	(2)	(1)
Curtailments		(3)	0	0
Settlements		(17)	(3)	(3)
Total recognized in AOCI and regulatory assets (liabilities)		172	(3)	(824)
Defined benefit plan, amounts recognized in OCI, before tax		169	3	(401)
Defined benefit plan, amounts recognized in regulatory assets, before tax		3	(6)	(423)
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial (gain) loss		80	(232)	166
Amortization of actuarial loss		(45)	(66)	(61)
Current year prior service cost (credit)		0	0	0
Amortization of prior service (cost) credit		179	186	188
Curtailments		0	0	0
Settlements		(1)	0	0
Total recognized in AOCI and regulatory assets (liabilities)		213	(112)	293
Defined benefit plan, amounts recognized in OCI, before tax		107	(55)	168
Defined benefit plan, amounts recognized in regulatory assets, before tax		$ 106	$ (57)	$ 125