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, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, amounts recognized in regulatory assets, before tax	$ 19
Pension Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial loss (gain)		$ 941	$ 538	$ 635
Amortization of actuarial loss		(512)	(414)	(629)
Current year prior service cost (credit)		0	68	(4)
Amortization of prior service (cost) credit		(4)	0	(2)
Curtailments		0	(3)	0
Settlements		(14)	(17)	(3)
Total recognized in AOCI and regulatory assets (liabilities)		411	172	(3)
Defined benefit plan, amounts recognized in OCI, before tax		271	169	3
Defined benefit plan, amounts recognized in regulatory assets, before tax		140	3	(6)
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Current year actuarial loss (gain)		22	80	(232)
Amortization of actuarial loss		(49)	(45)	(66)
Current year prior service cost (credit)		(111)	0	0
Amortization of prior service (cost) credit		124	179	186
Curtailments		1	0	0
Settlements		(1)	(1)	0
Total recognized in AOCI and regulatory assets (liabilities)		(14)	213	(112)
Defined benefit plan, amounts recognized in OCI, before tax		6	107	(55)
Defined benefit plan, amounts recognized in regulatory assets, before tax		$ (20)	$ 106	$ (57)