Employee Benefit Plans (Net Periodic Benefit (Credit) Cost and Amounts Recognized in Other Comprehensive Income and Regulatory Assets and Liabilities) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Service cost		$ 173	$ 162	$ 157
Interest cost		351	394	337
Expected return on plan assets		(777)	(708)	(663)
Amortization of prior service (credit) cost		1	1	1
Amortization of net actuarial loss		206	172	193
Settlements, curtailments and special termination benefits		14	72
Net periodic benefit (credit) cost		(32)	93	25
Changes in plan assets and benefit obligations recognized in other comprehensive income and regulatory assets and liabilities:
Current year net actuarial (gain) loss		166	16	490
Settlements and curtailments		(81)	6
Amortization of net actuarial loss		(206)	(172)	(193)
Amortization of prior service credit (cost)		(1)	(1)	(1)
Total recognized in other comprehensive income and regulatory assets and liabilities		$ (122)	$ (151)	$ 296
Significant assumptions used to determine periodic cost:
Expected long-term rate of return on plan assets				8.75%
Weighted average rate of increase for compensation		4.23%	4.20%	4.09%
Pension Benefits | Minimum
Significant assumptions used to determine periodic cost:
Discount rate		2.77%	3.57%	3.80%
Expected long-term rate of return on plan assets		7.00%	7.00%
Crediting interest rate for cash balance and similar plans		2.31%	2.77%	3.00%
Pension Benefits | Maximum
Significant assumptions used to determine periodic cost:
Discount rate		3.63%	4.43%	3.81%
Expected long-term rate of return on plan assets		8.60%	8.65%
Crediting interest rate for cash balance and similar plans		2.83%	3.63%	3.01%
Other Postretirement Benefits
Service cost		$ 28	$ 26	$ 27
Interest cost		58	68	56
Expected return on plan assets		(156)	(140)	(143)
Amortization of prior service (credit) cost		(49)	(52)	(52)
Amortization of net actuarial loss		6	10	11
Settlements, curtailments and special termination benefits		(59)	42
Net periodic benefit (credit) cost		(172)	(46)	(101)
Changes in plan assets and benefit obligations recognized in other comprehensive income and regulatory assets and liabilities:
Current year net actuarial (gain) loss		(110)	(98)	78
Prior service (credit) cost		(6)	2	(4)
Settlements and curtailments		59
Amortization of net actuarial loss		(6)	(10)	(11)
Amortization of prior service credit (cost)		49	52	52
Total recognized in other comprehensive income and regulatory assets and liabilities		$ (14)	$ (54)	$ 115
Significant assumptions used to determine periodic cost:
Discount rate				3.76%
Expected long-term rate of return on plan assets		8.50%	8.50%	8.50%
Health care cost trend rate	[1]	6.25%		7.00%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate)	[1]	5.00%	5.00%	5.00%
Year that the rate reaches the ultimate trend rate	[1]			2022
Other Postretirement Benefits | Minimum
Significant assumptions used to determine periodic cost:
Discount rate		3.07%	4.05%
Health care cost trend rate	[1]		6.50%
Year that the rate reaches the ultimate trend rate	[1]	2025	2023
Other Postretirement Benefits | Maximum
Significant assumptions used to determine periodic cost:
Discount rate		3.52%	4.41%
Health care cost trend rate	[1]		6.60%
Year that the rate reaches the ultimate trend rate	[1]	2026	2025

[1] Assumptions used to determine net periodic cost for the following year.