Summary of Significant Accounting Policies (Schedule of Weighted Average Depreciation Rates) (Detail)	12 Months Ended
	Dec. 31, 2021	[1]	Dec. 31, 2020
Generation
Composite weighted average depreciation rates for utility plant	2.46%		2.50%
Transmission
Composite weighted average depreciation rates for utility plant	2.56%		2.56%
Distribution
Composite weighted average depreciation rates for utility plant	2.48%		2.42%
Storage
Composite weighted average depreciation rates for utility plant	2.85%		2.75%
General and other
Composite weighted average depreciation rates for utility plant	2.27%		3.17%

[1] (1) Rates include the impact of a change in depreciation rates approved in connection with the settlement of the electric base rate case, which resulted in a decrease to depreciation expense of $6 million and an expected annual decrease of $12 million.